What is a healthy fish?

As responsible hobbyists we should all want to purchase and keep healthy fish and yet fish health can be a very contentious topic. If we all want the same thing, why do we argue about this topic so much? I feel it is because there is no easy answer to the question of when to consider a fish “healthy”.

Healthy, according to Merriam-Webster, has the following definitions:

1a: free from disease
1b: not displaying clinical signs of disease or infection
2: beneficial to ones physical, mental, or emotional state : conducive to or associated with good health
or reduced risk of disease
3: showing physical, mental, or emotional well-being : evincing good health

There, now I can rest easy because we all know what a healthy fish is. Ok, maybe not. I do think we can all agree that the end goal is to have fish that meet the 3rd definition by showing physical well-being. The problems arise when we look at the first 2 definitions. To explore how these first 2 definitions cause conflict, I want to address it from the perspective of a fish and the common parasite Cryptocaryon Irritans (Marine Ich).

Can a fish with Cryptocaryon Irritans be “free from disease”? Cryptocaryon Irritans is a pathogen which means it can be the causative agent of a disease. A disease is defined as “the condition of the living animal or one of its parts that impairs normal functioning and is typically manifested by distinguishing signs and symptoms”. A pathogen only becomes a disease when it reaches a density or level of damage that causes symptoms. We know fish can act as hosts of Cryptocaryon Irritans while not showing symptoms so yes, a fish with CI can be “free from disease”. Definition #1a and b can be met for fish with and without Cryptocaryon Irritans.

What about definition #2? Is carrying Cryptocaryon Irritans “beneficial to ones physical state” or “reduced risk of disease”? As an obligate parasite, Cryptocaryon Irritans will do at least minor damage to a fish even if its immune system prevents that damage from reaching the level of a disease. It is also true that a fish without Cryptocaryon Irritans has no chance of it becoming a disease. That is about as reduced a risk as it can get. Based on this, a fish without Cryptocaryon Irritans would be healthier than a fish that carried it. Or would it?

To explore this further I am going to veer away from fish health and move into the world of human health. Is a person who has just received a measles vaccine healthier than a person without it? The Measles vaccine works by injecting a weakened strain of measles (pathogen) into our body so that our immune system can learn to fight it with a low risk of it reaching the level of a disease. Is a freshly vaccinated person who still has a weakened strain of Measles be considered healthier than a person with no Measles at all? At face value, no, they wouldn’t be, but that doesn’t tell the entire story. We vaccinate for a reason. It builds an immune response so that if a more virulent strain of Measles is encountered in the future the vaccinated person has a higher chance of not having disease symptoms than the person who is unvaccinated. From this perspective, exposure to a parasite does offer a condition that is “beneficial to ones physical state” and a “reduced risk of disease”.

Pathogen exposure in a fish does act much like a continuous vaccine. Instead of a lab weakened pathogen the immune system of the fish weakens the pathogen. This prevents the transition from a pathogen to a disease. The fish immune system is unlikely to completely eradicate the pathogen allowing for the continuous exposure needed to maintain the immune response. What is a healthy fish? Quite simply, a healthy fish is one that is not diseased, which means it is not showing symptoms of a pathogen. Fish with or without certain parasites and pathogens can both meet this definition. If we were to ask which is healthier, a fish with or without pathogens, the answer is less clear. I would argue that the fish that is free of pathogens is healthier in every aspect except future risk.

So why does a pathogen become a disease? This circular diagram does an excellent job showing the relationship that is likely to cause a pathogen to transition into a disease. One factor is how virulent the pathogen is. This gives consideration to how fast the pathogen reproduces and how much damage it can do. Gram negative bacteria tend to be more virulent than gram positive bacteria. Amyloodinium Ocellatum (Marine Velvet) is more virulent than Cryptocaryon Irritans. Another aspect is how susceptible the host is to each particular pathogen. Clownfish are more susceptible to Brooklynella than Angelfish are. Wrasses are more resistant to Cryptocaryon Irritans than Acanthurus Tangs are. The health of the fish also comes into play. Is it injured or suffering from malnutrition? Does it have genetic issues that reduce its immune system response? And then there are the environmental conditions or stressors. How is the water quality? Is the system temperature stable? Does it have aggressive or incompatible tank mates? Is the tank over or under stocked? These all come into play for a diseased fish.

I think it is important to emphasize the definition of a disease - the condition of the living animal or one of its parts that impairs normal functioning and is typically manifested by distinguishing signs and symptoms. As the circular graph shows, a fish can have symptoms of a disease without a pathogen being present. Stress alone, in a fish that is susceptible to stress, can impair normal functioning behaviors of that fish. A wrasse may not be considered susceptible to Cryptocaryon Irritans but with enough stress and a high enough number of parasites in the system a wrasse can show disease from the parasite. An extremely virulent pathogen combined with a susceptible host can lead to disease even in a stress free environment. And finally, if you have a susceptible host, highly virulent pathogen and adverse stressors, your fish is almost guaranteed to have those pathogens become a disease.

How do we reduce how virulent a pathogen is? The average hobbyist isn’t going to genetically modify that pathogen to make it less virulent as is done for vaccines so we can cross that one off the table. One option we do have is to eradicate as many known pathogens as possible. A pathogen that isn’t present offers no risk to the host. Copper and Chloroquine Phosphate can eradicate Cryptocaryon Irritans and Amyloodinium Ocellatum. Praziquantel can eradicate flukes, Turbellarians, and some internal parasites. Metronidazole can eradicate Brooklynella, Uronema Marinum, and other internal parasites. For this to be effective it is important to quarantine everything and prophylactically treat every fish that goes into your system. Remember, just because a fish isn’t showing symptoms does not mean it isn’t a carrier. Just like all things in life, prophylactic treatment does have its down sides. It requires a separate hospital tank to treat fish. It requires medications which are not available in many parts of the world without a veterinarian’s prescription. It requires quarantining everything wet, not just fish, as fish pathogens can be introduced via inverts and corals. If your bio-security fails then you have to remove and retreat every fish along with a fallow period to re-eradicate the pathogens. Chemical treatments are stressful on fish and weaken their immune system so while we can reduce the threat from the target pathogen we are increasing their vulnerability to other pathogens.


We do have other options to reduce the virulence of pathogens other than just eradication. We can reduce the number of free swimming parasites by installing UV filters. They must be properly sized and have the correct flow rate for the application. Ozone systems can be used to kill parasites and bacteria that pass through the filtration system. Diatom filters will mechanically remove free swimming parasites to help reduce the amount that can re-infect fish in the system. Free swimming parasites can be consumed by corals if they are captured. Peppermint shrimp have been proven to eat the eggs of some flat worm related fish parasites. Our closed systems have some disadvantages compared to the open ocean but we should do our best along these lines.

What can we do to reduce environmental stresses? Maintaining good water quality is important. This doesn’t mean we have to do large water changes although that is one option. We want to make sure our biological filter is well established prior to adding fish to prevent toxic ammonia from building up. A properly set up skimmer will help keep water clean. Activated carbon and/or hydrogen peroxide help break down and remove organics from the water column. We can use redundant heaters on controllers to help minimize the chance of a temperature spike. We should have a plan on how to keep our systems stable during a prolonged power outage. We should ensure we have spares of our critical components to keep the fish alive and healthy if we experience an equipment failure.

We also need to consider how we stock our tanks. With some exceptions I feel we should stock our tanks from smallest to biggest and from most peaceful to most aggressive. The size of our systems should dictate which fish are and are not appropriate. Even a Dwarf Angelfish will not do well in a 2.5g nano. We should consider fish compatibility. It may not work to stock peaceful fish with highly aggressive fish. Some species of fish will not tolerate others and some fish are intolerant of conspecifics (same species).

What can we do to reduce the susceptibility of the fish to act as a host to pathogens? The impact of gut microbiota on the fish immune system cannot be overstated. Ideally, we would all feed fresh, never frozen seafood straight from the ocean. This isn’t realistic for most of us. Regular feedings of high quality frozen food can boost fish health. If you don’t feed live foods, it is worth considering using foods with probiotics added to them. Probiotics are now available in some frozen and pellet foods. Not every feeding needs to use them, but consider using them at least on occasion. A variety of probiotics would be even better as they each may have a different way of impacting the fishes immune system.

Visual observation can also play a part. You may be able to determine if a fish is showing signs of a disease if you can monitor it prior to purchase. If you order from an on-line supplier a QT can provide a chance to do an observation after receipt. It also allows a potentially weakened fish the chance to feed and build health before having to compete with tankmates for food and space.

My desire would be that everyone in the hobby would take a holistic approach to fish health. We would all design redundant systems designed around good filtration and steady parameters. We would never experience those challenges in life that cause our husbandry to slip. We would stock fish appropriately and not put an Annularis Angelfish in a 75 gallon system or put 3 Acanthurus Tangs in a 150g system. We would all stop by the ocean on our way home from work to collect food for our tanks each day. Obviously, this isn’t going to happen. Instead, I’ll ask this. Keep these ideals in the back of your mind as you build and stock your systems. Be honest with yourself and how you will maintain and feed your system. Think about the circular diagram and how susceptible your tank will be to disease based on your plans. Ideally, we will be as far from the center of that chart as possible. If we succeed in that, we will likely never have a fish disease issue.

*All pictures used in this article are mine and used with my permission.
About author
Steven Frick has spent much more time under the ocean than keeping a small piece of it. He got his start in the electrical field in the US Navy Nuclear Power program as an electricians mate. After 5 years of service on the submarine USS Henry M Jackson he finished his final 3+ years of service teaching electrical theory at the Knolls Atomic Power Laboratory. Currently, he runs the projects and maintenance for the power distribution system of one of largest electrical consuming heavy industrial companies in the Southeastern United States. He wrote his sites electrical safety policies and routinely acts as a consultant to other industrial facilities looking to improve their electrical safety programs. As someone who loves to both learn and teach, he has focused his attention on his newest hobby, reefing.

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As a new reefer I am grateful for this point of view. I know how much of a rabbit hole things on forums can be, and this alleviates a lot of the stress of worrying if I'm doing the right thing. Thank you for this.
Prefer to listen to my vet who has a completely different view. He went to school for 8 years for veterinary medicine.

Ich kills fish all the time. These boards are filled with those stories. It’s pretty insane to think that fish with ich parasites are “healthy” and that confined glass boxes filled with parasites are somehow “healthy environments”

I get that many people are successful in keeping tanks with ich in them. But be honest about it. You are just managing a disease that is insanely simple to cure.

I’ve also seen tanks that managed ich for months or years and then one day They are completely wiped out by one small trigger that brings on a ich-pocalypse. That’s not healthy either.

Be honest thread.
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I never made any recommendations about treating or not treating Ich. This tells me you came with a very biased opinion and an agenda.

I also don't think your vet would appreciate your throwing him under the bus by saying he claims a fish with Ich cannot be healthy. A simple google scholar search of "immune response to cryptocaryon irritans" will quickly show that he is denying science.

Final thought in light of the recent Columbus Day holiday. A bunch of healthy Europeans came to the new world and brought with them smallpox and other diseases that devastated the native populations. Despite what you and your vet think, this did not make the Europeans unhealthy.
Great article @Brew12. Easy to understand and very informative.
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You can’t just throw out the first part of a definition of a word and claim that sick fish are somehow “healthy” because they partially meet the things in one of the lower definitions.

Most people don’t consider a fish with marine ich as “envincing good health” just like most people wouldn’t think a dog with heartworms is “healthy” even though they might look fine. Their bodies are battling a parasitic invader in both cases.
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I the requirement for healthy was not fighting off parasitic invaders no one would ever be considered healthy. Our intestines are full of parasitic invaders that our immune system keeps in check.
Your heartworm example falls short as it is not a parasite that a dogs immune system can manage in a way that doesn't harm the dog. Cryptocaryon Irritans can be kept in check by a fishes immune system.
Thank you for writing such an informative and very comprehensive article.
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