They should be active and eating well. When my water chemistry is off, my tetra's are sensitive and quit moving. Know the type of fish also, some fish need companions and some don't. Some need plants and others do not. If your water is okay and not fluctuating in say ph or ammonia, then most fish will be just fine. I hope this helps.
I love my fish like people love dogs or cats, I think of them almost like children. I don't know if I can tell when they are happy but my opinion is that a healthy fish is a happy fish. Make sure water quality and temp are perfect. Feed a variety of foods, not too much and not too little. Provide interesting habitat that resembles the fishes natural habitat. Keep a close eye on your fish to quickly spot any sign of disease or parasites. Also, whatch out for signs of aggression- no one is happy if they are being bullied. Make sure the tank is big enough to accommodate your fish with extra room to spare. Above all , love your fish, I'm convinced they know and that they love back!
Good luck and thanks for really caring about your fish.
If they're swimming freely, eating like they'll never see food again (typical), not hiding out or looking stressed, and hanging with their same type for the most part, then I would say that the fish are as happy as a fish can be happy.
Keeping them in the best environment possible by doing your weekly water changes will keep them healthy so - happy
As others have stated..it's pretty easy to tell if they are happy....
In my tank..IF... the light is on and they DONT see me.. I can see them searching the bottom and plants for "goodies" the guppies are chaseing one another... the lyre tail barb is pestering anyone he can... the rasboras are zipping around in tight formation and the angel is snooping around "her" amazon Sword and "cory" the albino cat is "ferreting" around the drift wood stump... but if they see me.. every last one of them DEMANDS to be fed and starts millling around in front of the tank acting like my dog does when he hears the can opener!
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.
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