Clean water!! Make sure your parameters are spot on (0 ammonia, 0 nitrite, 5-10 nitrate) and the temps are correct (78-82). I bought a very, very ammonia poisoned fish from Petco a few years ago (curse those small cups) - and he's still very healthy and happy because I've had him in a cycled 5gallon this whole time. Fish are very hardy creatures, but you have to be diligent with water parameters.