Shoaling aquarium fish: choosing a group size

The biggest single study held zebrafish for nine weeks at five densities, from one fish per liter up to sixteen, and measured cortisol, aggression, distribution and growth together. Welfare was compromised at one fish per liter and nowhere else: those fish put more cortisol into the water per fish, pushed higher in the tank and fought more often than any of the crowded groups³. The authors' conclusion is that zebrafish can be held anywhere between four and sixteen fish per liter without compromising their welfare³.

Read that the right way round, because it is the opposite of what the hobby worries about. Too few was the measured problem, and the band that cost the fish nothing ran from four per liter all the way to sixteen.

The bottom of the range has been probed once more, and the result is the most useful number on this page for a small tank. Zebrafish in groups of four held themselves against a current better than zebrafish in pairs, and each fish in the four spent less time out in front, which is the position that costs the most to hold⁴. The improvement appears between two fish and four. Not at six, and not at any other round number.

And a fish does not use a constant anyway. Preferred group size shifts with how hungry the animal is, with what it has just been frightened by and with where it is standing⁵. That is the real objection to the rule. Six is not too low or too high. It is the wrong kind of answer.

The last two rows are the only published studies that ask the hobby's own question, in the hobby's own animals, and both are behind paywalls with no abstract published for the first of them⁶⁷. Two papers, standing under every stocking recommendation ever written. Anybody with a university library login can move this whole subject forward in an afternoon.

One more thing about cories, because it is the group most often sold two at a time. They shoal and they almost never school. They forage together, rest in contact and go up for air in a loose rush, and they will not line up for you no matter how many you buy. A buyer told to expect schooling sees none of it, decides the fish does not care, and takes two home. That is the single most expensive misunderstanding in this whole subject, and it comes straight out of using one word for every social fish in the shop.

There is also good evidence that how a young cory was raised follows it into adulthood. Juveniles reared with adults present, against juveniles reared only with their own age group, behaved differently as subadults: the same-age fish nudged each other more, were less active at rest, froze for longer after a startle and bunched together harder when frightened¹². The authors put it down to the group security the fish had while growing up. Which means farm-raised juveniles from a single-age cohort are not quite the animal the wild accounts describe, and twelve bought together are not the same social object as six now and six in the spring

Shoaling evolved for five reasons, and four of them are switched off the moment the fish is in a tank. There is no predator to confuse and none to be diluted among. The measured energy saving needs fish swimming hard in one direction at a sustained speed⁹, which a group of tetras hovering over a carpet is not doing. Shared watching still happens, but it is watching for nothing.

Point that out and the usual argument for buying a group falls over. Here is the one that does not. The behavior is not a calculation the fish runs each morning. It is a disposition built in over evolutionary time, and it goes on running in an empty white box with no threat in it, exactly the way it runs in a river. A fish kept alone does not become a fish that does not need company. It becomes a fish doing a social behavior with nobody to do it with.

What isolation does to the animal has been measured, and the honest version is stranger than the folklore. Zebrafish kept alone for six months came out with lower whole body cortisol than grouped fish, not higher, alongside clear changes in serotonin and dopamine metabolism¹⁴. Isolated juveniles ate more and converted it worse, expressed more of an inflammatory gene, and grew narrower intestinal villi¹⁵. So anybody who tells you a lone shoaling fish is stressed and means a cortisol reading is over-claiming, and anybody who says it is fine because its cortisol is normal is wrong in the other direction. The animal is measurably changed, down to its gut lining. Stress is simply the wrong word for it.

There is a good reason to buy a shoal that all looks the same, and the shop will never give it to you, because variety inside a group is the thing being sold. When one individual in a group looks different from the rest, predators single it out, and the whole group loses some of its protection because that fish gives the hunter a way through the confusion¹⁶¹⁷. Oddity is not a private problem for the odd fish.

Fish sort themselves accordingly. Size oddity raises predation risk in minnow schools, which is why schools segregate by size in the first place¹⁸, and large guppies given the choice picked a shoal that matched their own size over a shoal that was simply bigger¹⁶. Assortment by species, by size, by color and by parasite load are all documented¹⁶.

Every one of these is a normal Saturday in a shop: six neons and one albino neon, a shoal of cardinals with two rummynose for variety, five small cories and one big one, a bag of assorted platies, one long-finned individual among five standard ones. Nothing in a tank is going to eat any of them. What is left is the avoidance, which shows up as the odd one held at the edge of the group, or chased, or simply never joined.

The caveat belongs right here rather than in a footnote: nobody has measured the welfare of an odd fish in a predator-free aquarium. The step from predators pick out odd fish to the odd fish in your tank has a worse time is an inference. It is a well-founded one, because avoidance is the part that was under selection, and it is still an inference. Buy one species, at one size, in one visit. It is a better tank and a cheaper bill.