Aquarium Myths and Misinformation

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Aquarium Myths and Misinformation

c. 2023 Jay Hemdal

Over the last 50 years, I’ve heard a number of myths and possible misinformation applied to aquariums and the husbandry of the animals contained in them. Most of these seem to go unchallenged; at least they keep showing up online, long after they have been shown to be incorrect.

Some of these myths take on the same characteristics as “urban legends” (Such as the story of the child who gave her hamster a bath and then used a microwave oven to dry it off – with explosive results!). These are stories that people hear and believe “that it must be true because their friend knew a person whose sister actually saw it happen”. The true source of these stories can never actually be tracked down, and most are not based in fact – although the most successful myths and legends always have a small kernel of truth to them.

Other myths arise from over-extrapolation from a respected source, like a scientific publication. Some people will read the abstracts of scientific journals and then try to apply the conclusions to aquariums. This does not always work as intended. There was one paper published that showed that copper sheeting killed Cryptocaryon tomonts in aquariums on contact. Some people thought - Aha! a new ich treatment! If you carefully read the article however, you would have noticed that the researchers changed 100% of the aquarium water, twice a day in order to keep the free copper levels low enough not to harm the fish.

I’ll admit the sticking point here is that I’m asking you to accept my de-bunking of these myths with just a little more evidence than is present in the myths themselves! I also realize that not everyone will agree with my assessment of some of these issues. I do ask those who do disagree, to reflect on why they do, some of these beliefs are so firmly entrenched in the hobby that people tend to hold them a lot more dearly than the evidence would indicate they should. At the very least I challenge you to always question unsubstantiated claims, they may turn out valid in the end, but many of them are eventually relegated to the realm of myths. Remember, it is the responsibility of the person making fantastic claims to prove their statements are correct, not the other way around. For example, you might hear somebody claim that they saw a U.F.O. If you don’t believe them, it is not your duty to prove them wrong, they must supply evidence to support their claims.

76 day fallow period (or longer) for marine ich, Cryptocaryon: Published in a paper where the co-author was also the editor for the journal. The original study was from a PhD thesis that isn’t widely available. 45 to 60 days at 81 degrees F. is a more reasonable, but still effective fallow period. Fallow periods for other disease are different.

Adding vitamins or food additives like Selcon, ginger or garlic to the aquarium’s water: This just feeds the heterotrophic bacteria. Aquatic animals, if they uptake it at all, do so slower than the bacteria do.

Aquariums can hold a set number of inches of fish per gallon: You see this time and time again; “An aquarium’s holding capacity is 2” of fish per gallon”. Sometimes the number differs, in other cases the author adds refinements to the equation to take into account the aquarium’s surface area, filtration method, even water temperature. In every case, the equation has the same fatal flaw: With all else being equal, the waste output of fish, and their subsequent bio-load is not based primarily on the length of the fish, but more so their mass. An easy way to see this is to try to visualize the common formula of “two inches of fish per gallon”. Certainly, a ten gallon aquarium can safely house 20, one inch long guppies. Now, even though the “inches of fish” are equal, try adding one 20” long Pacu to the same tank and see what happens! As the length of a fish increases in a linear fashion, its mass, (and subsequent bio-load) increases by the length raised to the third power, multiplied by a constant (which varies for differently shaped fish). Another variable which also affects bio-load independent of the length of the animal is a given species innate metabolic rate; some species are more active, consume more food and thus produce more waste products than others of the same size.

Avoiding temperature shock is the main reason for acclimating fish: A variety of acclimation techniques have been presented over the years, all with the hoped-for result of reducing stress and shock to an aquarium animal being moved from one aquarium to another. Some people have embraced this idea and taken it to the extreme. There was the aquarium keeper who would float bags for fish in his tank for eight hours rather than the 30 minutes recommended by the pet store; the reasoning given was “in order to expose the fish to the slowest possible change in temperature”. Never mind that in this case, the water temperature in the bag was fully equilibrated with that of the aquarium within the first ten minutes. Even if fish were exposed to a sudden change in water temperature, just how harmful would that be? As a SCUBA diver, watching fish swimming up and down along a ten degree thermocline with apparent impunity makes me wonder. I’ve even seen a damselfish diligently guarding its nest along an 8 degree thermocline in the Galapagos Islands. The continually changing water temperature affected neither the fish nor its eggs. Overly long acclimation times do more harm than good. Take the example of fish, which have been in their bags for over 24 hours. Typically, the pH of their water will be very low, while the ammonia concentration will be high. As it turns out, ammonia is much less toxic to fish at a low pH. Imagine what happens to those fish during a lengthy acclimation; as the carbon dioxide is driven out of the shipping bags, the pH rises. Even though the aquarist is slowly adding tank water to the bag, it isn’t happening fast enough to dilute down the ammonia before the rising pH makes the ammonia toxic enough to kill the fish right in the bags. In other cases, the stress imposed on certain shy fish by restraining them in a clear bag at the surface where they cannot hide is greater than just releasing them so they can seek a secure hiding place. Acclimation times of more than 30 minutes are unnecessary and potentially do more harm than good. The only criteria that MUST be managed carefully is exposing fish to a rise in salinity/specific gravity. This must be done very slowly, or better yet, in stages, by changing the salinity of the tank itself.

Dietary supplements as a “medication”: The best diet in the world will not stop active infections – this is called the “chicken soup” syndrome. A proper diet is of course important for long-term fish health, it's just that changing to "great diet" will not stop active disease. Give this the logic test: will feeding your dog a great diet keep it from getting ticks or fleas? Of course not.

Drip acclimating shipped fish: Not if there is high ammonia. Best to match the temperature, pH and salinity and move the fish directly over and then acclimate them (with no ammonia) to your tank.

Feeding Peas to cure swim bladder issues: The recommendation of "feeding peas" has a long and convoluted history. It began as a treatment for issues with floating fancy goldfish. An aquarist at the Shedd Aquarium promoted this remedy back in the 1960’s and it does work. However, the issue with fancy goldfish is because they have weirdly shaped intestines that trap gas, making them float. Feeding peas works as a laxative, expelling the gas and allowing the fish to swim normally. At some point, this idea got "transferred" to marine fish with swim bladder problems. Feeding peas has NO benefit for swim bladder issues at all, yet the idea has been spread all over the Internet.

Fish only grow in size to fit their aquarium: Most intermediate and advanced aquarists, (or anyone who has ever purchased a Pacu) knows this statement is more accurately stated as: “Fish grow rapidly to a maximum size for that aquarium, then the growth rate slows a bit, but they still outgrow that aquarium if it is in their genetic makeup to grow that large”. Of course, the irony of this seems to be that this slowing of the growth rate only gives the aquarist time to save up money for a larger tank. When the purchase is made and the fish is placed in its spacious new quarters, its growth rate again speeds up until it again is about to outgrow its new tank!

Flying gurnards taking to the air: These bottom dwelling fish were pictured by early naturalist artists as being capable of gliding through the air like flying fish. In reality, these fish extend their huge pectoral fins in an attempt to make themselves look too large to eat by potential predators. As recently as 1976 an eminent fish taxonomist wrote that these fish are “supposedly capable of gliding short distances”. If one considers jumping out of a tank onto the floor as “gliding a short distance” then perhaps this is true, otherwise we should consider this myth dead and buried.

Flying hatchet fish: This is a rather “slippery” tale. Every once in a while, I’ll come across a reference regarding the flying ability of the South American hatchet fish. That’s right, I said FLYING not just gliding. Apparently, the argument is made that the deep breasts of these fish serve as anchor points for huge pectoral muscles, which the fish uses to vibrate its fins and achieve a sort of powered flight. One description I read told of a school of these fish taking to the air and passing by the observer’s head making a loud buzzing sound. Although these fish are well known for their ability to jump out of tanks, I’ve never seen any direct evidence of powered flight. Perhaps they need a longer runway for takeoff? I’ve toyed with the idea of adding a school of these fish to a 20,000 gallon reservoir, and then give them a scare and see if they will fly under those conditions.

Huge fish thriving in a small aquarium: From time to time, I get reports such as a three foot long snakehead living in somebody’s one foot wide by four foot long 55 gallon aquarium. As I’ve previously mentioned, the “fish growing only to a size to fit the tank” statement is a myth, as these reports seem to bear out. Either these fish have an amazing ability to survive in horrendously undersized aquariums, or people’s ability to judge the true size of their fish is impaired, or a combination of both. That recalls the story of the “Florida tape measure”. It seems that some fish dealers have in their possession a magical tape measure: nurse sharks always tape out at 14” – even if they measure 24” when you pick them up at the airport. For other species such as angelfish, the same tape measure that the collector used on the nurse shark will give them a measurement of 8” for a show-sized queen angelfish that ends up being about the size of a half dollar when you unpack it.

Iodized salt is harmful to fish: After selling non-iodized aquarium salt in pet stores for 15 years, I strongly disagreed with the person who first proposed to me that iodized table salt worked just as well as the more expensive “aquarium salt”. Everyone “knew” that the iodine would kill the fish. I had a difficult time believing that this basic tenet of freshwater aquariums was false, but it is. You can safely add the same amount of regular table salt to an aquarium as you would non-iodized aquarium salt.

Lee Chin Eng Natural system: I remember seeing the photographs of Mr. Eng’s tank in hobbyist publications in the early 1970’s and wondering why I could not re-create a similar aquarium. He was reportedly able to keep all manner of fish, corals and other invertebrates in aquariums with just a slow trickle of air. Mr. Eng has evidently passed away, so we only have his photographs to document his systems. I would urge people to study those photographs very closely; quite a bit can be deduced from them by careful observation: A lot of the photos show huge numbers of shrimpfish along with damselfish, clownfish and batfish. Anyone who has successfully kept shrimpfish knows that they do best if fed live mysid shrimp and cannot compete against other fish for live food. My strong opinion is that Mr. Eng added these shrimpfish a short time before taking the photo – and may have done the same with some of the more delicate invertebrates in the tank. Other pictures show corals placed in unnatural positions, showing no evidence of any in-situ growth. Many of the other organisms are hardy species (at least for the short term) such as feather dusters, carpet anemones and red starfish. My hunch is that Mr. Eng set up aquariums with live rock and hardy animals, and then just prior to being photographed, he “spruced the tanks up” by adding a few showy, but delicate species.

Mangrove seedlings help maintain marine aquarium water quality: I can’t recall how many dead and dying mangrove seedlings I’ve recently seen offered for sale, floating jammed into pieces of Styrofoam in dealer’s holding tanks. Keeping mangroves healthy in aquariums is not a simple matter; they are prone to developing scale insect diseases and require very high light levels. Even if the plants do thrive, the argument that they will actively remove organic waste products from the aquarium’s water is difficult to prove. In one case, a 200-gallon exhibit containing three large, actively growing mangrove trees had been set up for two years. It did have a fairly heavy bio-load of fish, but an orthophosphate reading of 7.5 mg/l was totally unexpected based on the supposed nutrient removal ability of mangroves.

Mixing medications with focus + food: This cannot work unless you calculate the dose properly. General Cure should not be dosed orally, as the two components have two different oral doses. This article discusses that:

Mortality caused by medications, years after application: Copper, formalin and cyanide have all been implicated in fish loss years after exposure – this is not borne out by histopathology or veterinary necropsy. Public aquariums all have comprehensive, proactive quarantine/treatment protocols, yet they have some of the longest-lived fish. I've done three studies since 1983 that all show that latent cyanide mortality (while often severe) always manifests itself within 30 to 50 days of exposure. Long term copper exposure has been shown to cause damage to fish, but this is ionic, not chelated copper, and the toxicity appeared during or shortly after the copper exposure.

Nitrite is toxic to marine fish: Ammonia is highly toxic to marine fish at a high pH, but the salts in the water completely de-toxify nitrite ions for marine fish. Nitrite IS deadly to freshwater fish, unless some salt is added to the water.

Predatory fish, which will only accept live foods: I remember in the 1970’s, thinking that lionfish would only eat living fish. Sure enough, all my lionfish would greedily accept “feeder fish” from the very start and would always steadfastly refuse anything else once they have become accustomed to that diet! This was really just a case of a self-fulfilling prophecy, I expected them to eat only goldfish and sure enough, that’s what they became accustomed to! There are a variety of reasons why non-living foods may be preferable; lower cost, ease of storage, ability to supplement with vitamins, etc. Can every species of fish be “trained” to feed on non-living food? It seems that almost any species can, if the required amount of effort is made. The basic trick seems to be to begin feeding the fish an acceptable living food, but very soon after, begin inter-mixing some freshly killed individuals of the same food. As soon as possible, switch the animal over to all dead food, then begin offering slivers of the prepared food made the same size and shape as the accepted prey item. This technique works very well for anglerfish, lionfish and even trumpetfish. In some cases, though, the training effort is simply too great (such as with shrimpfish, seadragons and leaf fish). I once had a report that a person had gotten their seahorse to feed on flake food using this method, but I never saw this firsthand, and would hate to create another myth by putting too much stock in that story!

Ramp up medications slowly: Too many fish die from disease if you take too long, dealers don’t do this, should you? (the only exception is salt when you are raising the salinity or the old ionic copper/citric acid solutions).

Red-rimmed batfish feeding on bananas: Years ago, a person wrote an article for his local aquarium club recalling his experience feeding bananas to a pair of red-rimmed batfish (Platax pinnatus). His argument was that since batfish are residents of mangrove swamps, they should be well adapted to feeding on plant material. By that same train of thought, lions should be well adapted for feeding on grass because they are found in the grasslands of Africa. In addition, the red-rimmed batfish is a resident of deeper coral reefs; it is the round batfish, (Platax orbicularis) which is found as juveniles in mangrove areas. I’ll admit, I have tried feeding bananas to red-rimmed batfish on three occasions (but have never gotten one to accept any). As the article goes on, the author reports that the fish grew to a height which was 2” taller than the aquarium he reported keeping them in – it’s possible that the entire article was some sort of practical joke. The danger is that this article still gets referred to once in a while, it is archived in an on-line computer database, and people keep finding it when they run a search for information about this species.

Small aquariums are inherently less stable than larger ones: I love this myth! It’s one that every aquarist “knows is true” and has been around since long before the advent of thermostatically controlled heaters but should have been dropped once those devices were developed. You see, the only parameter, which is more variable in smaller aquariums than larger ones, is the effect that ambient air temperature has on unheated water. Small tanks will cool down quicker and heat up faster than larger tanks. Once thermostatic heaters were invented, this simply didn’t matter anymore. Take the case where you have two aquariums, and the only variable is their size – they have comparable filtration and other life support equipment (appropriately sized for each tank). If you then compare a ten gallon aquarium, which is housing “X” grams of biomass per gallon and 100 gallon aquarium housing the same ratio of biomass to volume, the biological stability of the two systems will be identical. This myth got its start in unheated tanks, but two other underlying problems have fueled it over the years. Notice that I said that the two tanks need to have comparable life support equipment. Very often, smaller aquariums have cheaply made filters and heaters, which negatively affect the overall stability of the system. In addition, it is much, much easier to overcrowd a smaller aquarium than it is a larger one; adding an extra 2” long swordtail to an already over-crowded ten gallon aquarium might increase the total bio-load of the system by 20%. The same fish being added to a similarly crowded 100 gallon aquarium would increase the total bio-load by perhaps 2%. The relative instability of these smaller tanks can often be documented, but the root of the problem is not directly correlated with the size of the tank.

Snapping shrimp can break aquarium glass when they snap their claws: I first heard this when I was about 12, when I purchased a small snapping shrimp and was warned by the salesperson that they “have been known to crack the glass of tanks”. After adding the shrimp to a small aquarium in my bedroom, I had a rough couple of nights, jumping up to check the tank every time I heard a snap. The glass never broke, nor have I heard any firsthand accounts of this phenomenon (it’s always, “It happened to a friend of mine’s brother). There are reports of snapping shrimp stunning fish which swim by, so there is apparently some force behind their “pop”, just not enough to crack glass.

“Stray voltage” causes fish loss or health issues: This is a red herring, stray or induced voltage (typically < 50 VAC) has no measurable effect on aquarium fish, as they are not grounded, so there is no electrical potential. Stray voltage has been ruled out as a cause of HLLE. True electrical shorts can harm or kill fish, as well as people. All aquariums must be plugged into GFI circuits.

Tank Transfer Method (TTM): This method does work, but in my opinion, it works best for ich (Cryptocaryon) and not for egg laying flukes at all. It can be very rough on fish due to ammonia and excessive handling, and already stressed fish being housed in buckets. Remember that lateral viewing of new fish is a vital tool for diagnosis. Top down views in buckets or totes do not give you a good enough view of how the fish is doing. Then, just when is TTM useful given the logistics of treating diseases? If you use it as part of a quarantine process, you still need a single tank to run the fish through additional quarantine for flukes. If your DT develops ich and you want to use TTM, you can't do that and just put the fish back into the DT, the ich will still be active. That means you would run TTM, but then need to move the fish to another tank during the proper fallow period for the main display aquarium.

Ultraviolet Sterilizers eliminate diseases from aquarium water: Ultraviolet units have absolutely no effect on disease organisms, which live on the fish directly and do not need to leave the fish during some part of its life cycle (some protozoans, trematodes and most pathogenic bacteria). Other parasites such as larger protozoans and all multicellular parasites too big to be killed by aquarium-sized UV units. The expected benefits from a properly operating UV sterilizer would be a lowering of the free-floating bacterial and protozoan populations, as well as fairly good control (close to 100%) of these organisms when water flowing from one tank to another first passes through the UV sterilizer. Side-stream sterilization, where the irradiated water is returned to the same tank is was taken from is always less than 100% effective – too many of the target organisms are able to escape passing through the unit and are able to continue to reproduce.

Using RODI and no aeration for FW dips: Use aerated, pH and temperature balanced water in all cases. Tap water is fine!

Using black mollies as “canaries in a coal mine”: Really only screens for ich, Cryptocaryon, and can introduce euryhaline (brackish water) trematodes into the system, as well as internal parasites carried by the non-quarantined mollies.

About author
Jay Hemdal
Jay Hemdal has kept aquarium fish since he was four. He set up his first marine aquarium in 1968 when he was nine years old. He later worked part time for many years at various local retail pet stores and fish wholesale companies while he was living at home and then during college. After graduating from college with a degree in aquatic biology, he managed the aquarium department of a large retail pet store for five years until 1985, when he was hired as an aquarist/diver (and later department manager) for a large public aquarium. In 1989, he accepted the position of curator of fishes and invertebrates for another public aquarium, where he remains today. Jay has written over 200 articles and papers as well as seven books since 1981.

Jay has also written for the following publications.

Advanced Aquarist Magazine
Aquarium Fish International
Aquarium Frontiers
Comparative Parasitology
Drum and Croaker
Freshwater and Marine Aquarium
International Zoo Yearbook
Journal of Aquariculture and Aquatic Sciences
North American Journal of Aquaculture
Progressive Fish-Culturist
Today's Aquarist
Tropical Fish Hobbyist
Zoo Biology

Latest reviews

Pros: A thorough list of myths.
Cons: Some explanations I think could have more info or be more clear, or sources, but that's a relatively small criticism.
The Dietary Supplements thing is a minefield, I'll say that. I agree with the other post about how good diet can be a tool to promote health/ward off trouble.

But what I wonder is in particular the issue of natural fish diet vs captive. For example, I have a Foxface Rabbitfish that for a long time refused to take any algae. It does now. It is supposedly omnivorous with some preference for algae. I've not found much written about what happens to fish when not fed their natural diet. The Foxface now eats algae and frozen so it's getting what I think is close to its natural diet.
Pros: I always thought that the Lee Chin Eng Natural Tank was a 'setup.' It seemed too good to be true. And as such, things that are too good to be true, usually are.
Cons: The analogy of fish "infections" and a dog's tick or fleas is comparing apples to oranges IMO. I differentiate an infection (bacteria, viral) from a parasitic attack. So this section's discussion might be expanded to differentiate the two. I agree with the bolded section title in general.
I was surprised not to find Melafix and Pimafix addressed in the article. Maybe I just missed it.

I'd rewrite Dietary supplements as a “medication”:
Of course diet and supplements don't "medicate." I agree with the bolded section title, but the discussion under it, I think, could be modified. Supplements can improve immunity and health to resist infections. Supplements given to a sick fish can improve the fish's immune system. A healthy diet with supplements can maintain a fish's resistance to bacterial and viral infection (or just prevent their display), but can't always prevent a parasitic attack. A good example is an aquarium known to contain Mycobacterium marinum, but the fish s how no signs of being infected. Diet and supplement in regard to parasites: I have seen (eDNA) the parasite Uronema marinum (Uronema) in aquariums in which the fish show no signs of infection. I attribute this to pristine water, proper tank flow, low to no fish stressors, tank cleanliness, and a good immune system provided by proper foods and supplements.

I'd say this is an excellent article with perhaps the noted concerns.
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Jay Hemdal
Jay Hemdal
Sorry, I missed your review: I left two major topics out of this article - products that don't work as advertised (I don't want to be sued) and I left out much about reefs/coral myths, as I'm mostly a fish person. As far as the dietary supplement issue, you and I disagree somewhat as to the level of benefit seen. What I am trying to avoid though are the all too common comments that I see: "Just feed your fish well and ich will go away."...that's just a good way to end up with fat, dead fish (grin).
I will always want to read articles by such knowledgeable people.

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Jay Hemdal
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