Oh, that one! From what I have seen, there are a lot of quite good resources provided by the Russian-speaking fishkeeper communities in Israel… but that's probably because I don't know any Hebrew, so can't search for those
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I found that the pair would beat the crap out of the other one or two, if only 3 or 4 were kept (not necessarily right away, but 2-5 years after they had been in the same tank together). 6 is a better number to spread any aggression, otherwise it would have to be a pair. 6 probably wouldn't work in anything under 4*2*2ish ft (I think the OP tank will be fine), but I don't think that they should *really* be kept in anything smaller. I have seen them in a 3*1.5*2ish ft, where they spent their whole time going around in circles or not doing much; in a 4*1*1 ft, where a pair with young was fine; and in a 5*2*2 ft, where a group of 5-7 year old fish live without showing any signs of aggression and spending most of their time exploring and interacting with each other.Keyholes make for great centre piece fish. 3 or 4 would look fantastic. Definately wanting to own some in the future.
The dimensions matter more, the volume is almost irrelevant when it comes to aggression. In a 4 ft tank which is almost 2 ft front to back and almost 2 ft tall, 6 will probably be fine, in my personal experience. It is of course possible for this to be the case, in a 4*2*2, or even in a 6*2*2 or a larger tank. It very much depends on individual circumstances and individual fish. I do not know the exact dimensions of the OP's tank, but I do know that in any tank smaller than 5 ft long, there is a high possibility of 1-2 fish being harmed if only 3-4 are kept, which is why it is best to go either for a pair (preferably one where the fish picked each other themselves) or 6+. 3-4 fish is a bad number.I wouldn't fancy 6 in a tank that is roughly 220 litres though?