I often read "Don't overfeed". Is that just because it fouls water or are fat fish unhealthy? How can you tell if your fish is too fat? I have 2 fish, but one always gets more than her share because she's tamer. So I feel I have to feed more, just so the other can get some. Mostly it's live food, so the water doesn't get dirty, but I'm afraid one of them is getting pretty plump. Any guidelines? Thanks!