Alright, well I used to volunteer for a local public aquarium and I can explain fish vs fishes.
Fish is singular to a species. I'm not 100% certain if they can be within the same family (for example: Prochilodus have at least 7 different varieties; Insignis: red in the top fin, 9 stripes when small and up to 13 stripes when 16" or so; Taeniurus: no red in the top fin; same other qualities as above; Nigricans: black tail; Lineatus: no colour in any fins; and the list goes on.).
Fishes has to do with a plurality of fish species. So we could go to an aquarium and say that we saw 'many fishes' or 'many fish' if it had two of the same species somewhere.
So
Fish: Singular species of fish and a single fish in general; "I saw a school of fish."
Fishes: Many species of fish; "I looked into my tank and peered upon many fishes. There was a Phoenix, Tiger and Albino Bichir! The Bichir had to keep going to the surface because his gills are useless and it's evolved a lung!"
Anyway. You see the point at hand I'm sure.
If you have need of further explanation I'll be glad to pull out some of my 7 or so dictionaries for exact reference. I too am a bit of a word freak.
Pax,
Protean
