TwoJeffs has a hack that is supposed to stop some of the madness, although some people reported it didn't help. You can find it in his section:
http://www.moreawesomethanyou.com/smf/index.php?topic=2881.0XPTL over at insimenator.net has a Plastic Aquarium hack. It makes it so aquariums are always perfect, but sims can still watch and feed fish. But I haven't checked it it was updated (or needs to be) for Pets.