. My feeling after seeing that was some fish need companionship to not feel stressed.
Most of the fishes we have is fishes evolved and adapted to reefs. They are also colourful - not the camoflauge you can see in sand dwelling species. Why can fish that is bounded to reefs (or stony habitants as Malawi Cichlids) evolv as bright colours reef fishes and Malawi cichlids have. The answer to this is IMO at least partly that these fishes best defence for not be eaten is that they can hide very fast - the colours is not negatively affecting their ability to survive. However if they not quickly observe a danger and not seek cover fast enough - they will be short lived. One of the factors is - in an environment that´s normally full of other fish - if they can see other or not. If they not see any other - its a sign of danger - they automatically seek for hiding places. IMO - you are very right. When I was in the cichlid hobby we often had to treat individual fishes in hospital tanks. We did not do any blind tests but we saw that a fish we need to treat or was recovering from a fight normally survive and recover more quickly if there was other fish in the tank. We always had 10 - 15 guppies in the hospital tank
Sincerely Lasse