This is a very good observation/question. I just posted in this thread mentioning "aggression" and "play" and touched upon this. But it is worth stating, because there is a real misunderstanding among some in the hobby who think that the fish swimming, eating and even spawning means they are "happy" or "healthy." This is not the case. It is absolutely impossible to discern happiness because we cannot communicate with a fish to know. And in any case what does "happy" mean to the fish. Healthy falls in the same category. Like all animals, the primary instinct in a fish is to reproduce to continue the species. This is how evolution works. The fish will do everything it can to spawn, and the fish will tolerate whatever environment we force them into in order to achieve this goal. This can go on for considerable time, until something causes the fish to literally give up because it can no longer manage to withstand the problems caused by an improper environment. And "environment: here means the water parameters, water conditions, physical aquascaping, numbers of its own species and the other fish in the tank which may or may not be compatible. All of these factors affect the health of the fish through stress. We know that over 90% of aquarium fish disease is the direct result of stress. I don't know how many times I have read posts about sick fish where it is so obvious that the aquarist is responsible because the fish has beeen placed in an unsuitable environment and it has no option but to struggle to manage. Survive and thrive are very different.
