Clean Up Crew (CUC) or Food For Thought

Food for Thought
Potential Pitfalls of Using Utility Organisms (CUC) in Aquariums

Jay Hemdal


Aquarists often add “utility organisms” (animals not just for use in a purely decorative sense) to their aquariums to serve some beneficial function. These are often called CUC (Clean Up Crew). But do they always work? As the title of this article suggests, the answer is, not all the time. The topic of this article is, of course, meant to be thought provoking and controversial. Still, there is much truth in the title; many aquarists rely too heavily on the use of utility organisms to serve a housekeeping function in their aquariums.
When it comes to aquarium housekeeping, better results might be obtained by managing the aquarium’s environment yourself or through the use of technology. In any event, differences of opinion are always a good reason to examine the status quo.

Even the most basic biological control method used by almost every aquarist—biological filtration and the nitrogen cycle—has a major fault. Bacteria assimilate ammonia and nitrite and convert them to a less toxic compound, nitrate. This is certainly a case where a utility organism is serving a very basic, important function – or is it? In many reef aquariums, an increase in nitrate levels is unacceptable, and other means need to be found to control nitrogenous wastes.

Freshwater and marine aquarists alike use animals in their aquariums to serve some function or utility. Catfish clean up the fecal waste of other fish, right? Snails eat slime algae (but leave plants alone), and you can use cleaner shrimp to cure fish diseases. Have a problem with a pest animal in your aquarium? Just add another species to control it. It all sounds so simple, such a natural way to control your aquarium’s environment, much better than adding chemicals, using specialized filters or having to do the work yourself. In reality, there are many questions that need to be answered before applying these biological control techniques, and care must be taken or the overall problems may be made worse. This tongue-in-cheek adaptation of an old fable may ring true for many of you:

A few days after it was first set up, the front glass of the aquarist’s new reef tank became covered with an unsightly thin brown coating of diatoms. He called his pet store and asked if anything could be done to rid his aquarium of this pest. They suggested adding more live rock “to absorb nitrates” and happily sold him some special pre-cured Nabua-Rota ultra branch(1) rock at $18 per pound. Sure enough, after three months, the diatoms went away (having used up all the available silica anyway). A few months later, the aquarist despaired as he noticed that the live rock was now overgrown with red slime algae. Not wanting to tear the tank down, he called his pet store again and asked if anything could be done to rid his aquarium of this new pest. The pet store suggested more snails and sold him fifty super hemi-grazers (1) at $7 each and told him to turn his skimmer up and do more water changes. The slime algae gradually went away (in the amount of time it took to do about four 30% water changes). Eventually, most of the snails died, and soon the aquarist noticed a few odd, little, shiny, green balls growing in the crevices of the live rock. He looked this up on the Internet and discovered it was another type of pest algae known as Valonia. The pet store cautioned him that these algae could crowd out more desirable reef organisms and sold him a dozen “emerald charger”(1) Mithrax crabs to eat the Valonia. The Valonia eventually started to disappear, but so did a small fragment of ultra yellow Porites coral he had paid $75 for(2).
Undaunted, the aquarist bought some more coral to replace the Porites. At the base of the coral head he noticed a cute baby brown anemone. You guessed it, a few months later the aquarist was back at the pet store asking which animal he could add to his aquarium to remove this biblical plague of Aiptasia anemones. They recommended a “show size” Australian copperband butterflyfish. His pocket $189.95 lighter(3), the aquarist returned home and added the butterflyfish directly to the tank.
A few days later, while watching the fish nibble languidly on his Hammer coral, he noticed that it had a few white spots on its side. Worse yet, he saw the same type of spots on the pride of his collection, a male flasher wrasse that he had since the tank was first set up. A bit of research told him he had an infection of Cryptocaryon in his aquarium. After yet another trip to the pet store, he returned with a group of Hawaiian cleaner wrasse, guaranteed to clear this marine “ick” fast. He bought six because he needed this to be a sure-fire remedy as he was scheduled to leave on a four-day business trip in the morning.
Everything looked okay when he left for his trip. The butterflyfish was posturing nicely for the cleaner wrasse. When he returned, he expectantly hurried over to the tank but was overwhelmed by a foul stench. The water was milky white, and all the fish, now totally covered in white spots, lay dead, draped across the live rock, floating at the surface or caught in the overflow. White streams of mucus were rising from the corals, and dead bristle worms covered the bottom. In disgust, not even unpacking from his trip, the aquarist tore the tank down and started the arduous task of throwing everything away and then cleaning and re-filling the empty tank. A few days after the aquarium was again set up, he noticed a thin brown film on the aquarium’s glass…was the cycle doomed to repeat?

There are seven possible outcomes that an aquarist may experience after adding utility organisms to an aquarium. Obviously, all aquarists hope to achieve the first outcome, but most end up with results that fit in categories two, three, four or five.

1) Successful – Complete control of pest species with no negative effect.
One example of this was an instance where a freshwater Mbu puffer (Tetraodon mbu) was used to control Asian needlepoint snails (Melanoides sp.) in a public aquarium. The huge puffer was very tame, and after cleaning out one tank, it could easily be coaxed into a bucket and moved to a new snail-infested tank. As long as the aquarists remembered to remove it soon after it cleaned up the snails, it generally left the other fish in the aquarium alone. Integral to this success is the ability to remove the control animal after its work is done. In that way, you don’t have to contend with the additional biological load placed on the system by the introduced animal.(4)

2) Positive – Partial control of pest species is achieved with no major negative effect.
An example of this is the addition of a dottyback or Coris aygula wrasse to control bristle worms. While these fish will be seen eating the worms, most of the worms escape predation and remain alive deeper down in the gravel bed. The fish generally leave the other invertebrates in the aquarium alone, so no harm is done, and the aquarist gets a “warm fuzzy feeling” seeing the fish hard at work.

3) Neutral – No real control seen, but no negative side effect.
Beware of over extrapolation. An animal may eat a food item in the wild, but this does not necessarily mean that it will eat enough of the same item in your aquarium to control a pest outbreak. Adding a few Berghia nudibranchs to an aquarium in order to control Aiptasia anemones often gives this sort of result. The nudibranchs tend to become lost in the aquarium without eating many anemones, but at least no harm is done.
Even some algae-eating fishes fall under this category. While many fish species will target certain undesirable species of algae, remember that the energy conversion going up each trophic level is usually about 1:10. That is to say, for every 10 grams of algae eaten by a fish, only one gram is converted to living fish tissue (and thus “taken out” of the system) while the other nine grams of biomass are released back into the system through waste products. Some of this energy is exported from the system as carbon dioxide, but the rest remains available for other primary producers to use, such as another species of algae. Combined with the energy from any fish food supplied by the aquarist, the net energy of the system may actually increase with the addition of these herbivores, causing increased primary production (Hemdal 1981).

4) Coincidental – Control seen, but utility organism didn’t cause it.
This is commonly seen when aquarists add snails or other herbivores to control diatoms. Very often, aquariums go through a transitional growth of various algae species, with one dying out only to be followed by increased growth in another species. Brown diatoms are one of the first algae blooms typically seen in an aquarium, and the growth usually fades on its own. If this natural reduction in diatoms coincides with the addition of herbivores, the aquarist often thinks the introduced animals are responsible for controlling the algae.
Almost without exception, recommendations to use various control organisms are made without any support from controlled studies. Aquariums are highly dynamic systems with life at much higher concentrations than in natural systems. Populations of organisms rise and fall with much more vigor than they would in nature. The adage, “Just wait a minute, and things will either get better… or they will get worse,” really holds true with aquariums.

5) Control, but with some negative results
This can be seen in the aquarium of a person who keeps adding Turbo or Astrea snails to a tank to control algae and then when the snails die, simply adds more. This actually increases nutrient loading in the system as the dead snails are converted by bacteria into dissolved organic material. While the standing crop of algae may be reduced by the grazing snails, the nutrient loading rises and must be dealt with by additional water changes, increased skimming or the use of phosphate “sponges” and denitrifying systems. Another example is the addition of a butterflyfish to control Aiptasia anemones; the fish may well completely control these anemones, but once they have been consumed, it may turn to feeding on Zooanthids or other primary aquarium invertebrates.

6) Negative – Complete control not seen, some negative side effects.
An example of this is the use of Mithrax (emerald) crabs to eat and control Valonia algae. Mithrax crabs are known to eat coral in the wild and have been seen nibbling on live corals in captivity. While they will eat some of this algae, isn’t one of the basic tenets of reef aquariums, “Never rupture the cell walls of Valonia algae when removing them by hand”? We are all told this releases spores that can settle out as new algae cells. How can the crabs feed on these algae without rupturing them the same way?

7) Disastrous – No control seen, huge negative side effect.
This can sometimes be seen when aquarists rely on cleaner species (cleaner wrasse, neon gobies, Lysmata shrimp, etc.) to control epizootic infections. If the cleaner species is specific in its food habits and the disease that needs to be controlled isn’t on its “menu,” the infected fish may die. In addition, cleaner fish are often just as susceptible to disease as the primary fish. So, who cleans the cleaner? Even if the cleaner animal does feed on the parasite causing the problem, it may not work fast enough to control the multiplying population.

Ultimately, the main concern with all of these biological controls is that they are additive, not subtractive. It is difficult to bring a system to homeostasis by adding more energy to it (in the form of additional life) – kind of like regaining control of a skidding car by stepping on the gas; this might work for an expert driver, but for the rest of us, watch out! Consider other means of control that are subtractive instead. Water changes, increased protein skimming, chemical filtration or physical removal of pest species by the aquarist are all subtractive control methods you should examine. There are also some additive methods that don’t include the addition of live organisms, such as using calcium hydroxide to control pest anemones (Hemdal 1992) or the addition of calcium hydroxide or increasing the lighting to attempt to shift the standing algal crop from one species to a more desirable one.

Consider the lessons learned through human translocation of exotic species in natural systems. For thousands of years, people have taken their favorite animals with them into new regions. The possible outcomes of these introductions mirror the seven possibilities outlined above. Many biologists hold the opinion that there has never been a planned or accidental translocation that ranked better than point three (a neutral effect). In most cases, the outcome carried with it some pronounced negative effect on the natural system (e.g., carp, purple loosestrife and Eurasian milfoil).

The case of the zebra mussel (Dreissena polymorpha) deserves special attention. While at first it was obvious that the introduction of this species into the Great Lakes was an ecological disaster (with the ensuing massive changes in the trophic structure of the lake combined with clogging of water intake lines, boat hulls and the like), lately anglers have been heard lauding the zebra mussel’s ability to clarify the water of Lake Erie through their filter-feeding action. For some people, this change in the condition of Lake Erie has made it seem as if the presence of zebra mussels is actually beneficial. This is a very shortsighted assessment. While the transparency of Lake Erie has indeed increased (benefiting the smallmouth bass anglers), the water itself is not any cleaner, neither in terms of bacterial contamination nor organic chemical pollution. Indeed, zebra mussels bio-magnify some toxins, making the animals that feed on them more toxic to those that in turn eat them, such as people eating bass that have fed on round gobies (another exotic species) that in turn fed on zebra mussels. What does this have to do with our aquariums? Simply put, it is an example of an ecological change that, on the surface, seems to be beneficial but ultimately will result in a negative outcome.
Of course, some biological control methods obviously do work. It would not be such a popular husbandry technique if it always resulted in dire consequences. With many aquarists, “less technology, more biology” has become a favorite mantra. Still, please remember that, in many instances, an aquarium will reach homeostasis on its own –a sort of uneasy balance among the myriad organisms achieved through basic aquarium maintenance.


1 The names of live reef animals are sometimes modified or simply made up in order to increase the species’ marketability.

2 The emerald crab, Mithrax sculptus, has been reported to feed on Porites coral at a rate of ten polyps per minute (Colin 1978).

3 Estimated from a wholesale price of $45 plus shipping.

4 1986 Personal experience.


Colin, P.L. 1978. Caribbean Reef Invertebrates and Plants. T.F.H. Publications, Neptune City, New Jersey.

Hemdal, J.F. 1981. Energy in the marine aquarium. Freshwater and Marine Aquarium. 4(11):20.

Hemdal, J.F. 1992. The use of calcium hydroxide to control Aiptasia anemones. Freshwater and Marine Aquarium 15(4):122.
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: Some good examples on what not to do.
Cons: Very biased. I was actually surprised at the inconsistency between the matter of fact tone and what was said.
I really do not get this article. It is written to dishearten people that like to chose their animals for their potential.
From the very first, it is not a matter of if animals can help the reefer, but how they hinder him/her.
If I am to find this article useful , I am invited to forget about:
  1. snail cleanup,
  2. tang/rabbits on algae patrol,
  3. salarias and shrimp on leftover duty,
  4. sump algae,
  5. copepods for .... whatever they do to detritus before they become food ,
  6. starfish for detritus and sand maintenance or
  7. hermit crabs for their.... I do not know what they do, I never liked crabs of any sort.
It sounds like the writer feels about everything not purchased for aesthetics to be,,, undesirable. THe same way I feel about hermit crabs.

Finally this is what black and white absolutism does. If the only result that is worth doing is a perfect result, then this article is amazing. Any pitfall should just make us work on finding what we did wrong, how it can be improved, and only after we see no results after several attempts (by us and others) then throw hard working blue collar critters under the bus! :p:p;Joyful;Woot;Woot
Pros: Touches on a subject we rarely think too deeply about, the clean up crew. This article provides a rational way to think about the CUC and whether we should be so flip about shoveling all those animals into our aquaria.
Cons: None come to mind.
I have recently begun to wonder whether the perceived value of the clean up crew needs to be reexamined. This articles has started the process. Like many “good” ideas for managing saltwater aquaria, a relook is overdo for the logic and science behind the idea that having a clean up crew is a good idea. I am especially concerned that a purchase of a clean crew is more than likely the death sentence for these creatures. The average aquarist spends little time to understand these creatures’ needs let alone ensuring they are met. Most clean up crew animals likely starve to death because they must live off the land, which in the case of aquaria, is a barren waste land. When I start my “save the snail“ campaign, Jay will be a charter member.
  • Like
Reactions: velvetelvis
Pros: concisely explains some of the pitfalls of adding various clean up crew
Cons: None
Excellent article - easy to understand.
Jay offers a realistic and reasonable opinion to the old reefing fables of certain 'cleaner' organisms fixing larger underlying problems, such as pests/outbreaks and disease.

Article information

Jay Hemdal
Last update
4.25 star(s) 4 ratings

More in General Saltwater Aquarium

More from Jay Hemdal

Ultimate Corals