11-18-2013, 12:05 PM
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I think it's easier to tell if the fish is unhappy - I have a hard time differentiating between indifference and happiness in fish.
The answer is experience. Fish behavior isn't always easy to judge. One needs a good understanding of the fish's baseline behavior by which to compare it, which comes from keeping multiple specimens, in various settings with various stocks. In my experience tank size plays a large role in fish behavior, and while someone might find X, Y and Z about a fish keeping it in too small of a tank, for example, they may find that not to be true when kept in a larger tank. Same goes for stock combinations. Fish A might be peaceful when kept with fish B, but remove fish B and you may find fish A to be a terror.
Fish aren't robots so while we may be able to expect certain things from certain fish, it's not set in stone. Some fish just break the mold, no matter how they are kept. But as others said - swimming around freely, eating with gusto, spawning, good coloration - all things that indicate a "happy" fish.