Hey I don't know exactly what's being done for creatures in the new release; so I'm trying to be a bit more vague here. Also, I don't know whether or not I'm going over covered ground, but if I am then I'm sorry. (never enough time...)
I was thinking that if the fish 'problem' remains in new version that there might be a good way to balance it out while still keeping the humor in it. You could set all (freshwater) fish to [FISHITEM]&[VERMIN_FISH](provided these tags still exist)...
Except for Carp.
Leave Carp as it is (except maybe remove [BENIGN]), and make an entirely new line of killers based off of them.
Giant Carp: like a Carp, but size 9
Whale Carp: size 16
Blood Carp: dark red, add [LARGE_PREDATOR][SAVAGE][CARNIVORE] and maybe some nastier attacks
Death Carp: like Blood Carp, but [EVIL] instead of [SAVAGE].
Mud Carp: don't know how to do this one; the idea is that it could waddle out of the river from time to time to attack dwarves near the riverbanks, but then there's nothing keeping them from waddling across the entire map to get at you (but that might be a good thing, if they're really slow out of the water...)
Flying Carp: no, you didn't see this, move along.
Holy Carp(!!!): this one's a bit tricky... needs something to make it special beyond the [GOOD] tag.
Lava Carp: once again, don't know how to do this. Magma/Fire Men/Imps all have [NOBREATH]&[SWIMS_INNATE]... does this mean DF treats magma just like 12000 degree water?
I know that all this has probably been been said before;
but I'd at least like to have this in some of the the first player mods for DF 2010.