I wouldn't mind being my betta, spoiled bugger has a 4' tank atm that he only shares with bottom dwellers.
I'm not sure what wild fish I'd like to be... Probably some kind of deep sea fish that never sees people... Hopefully I'd be capable of some kind of bioluminescence too... That'd be cool...
Wait - do jellyfish count as fish?? If so I'd want to be some kind of glowing deep sea jellyfish so I could bloob around glowing awesomely.