RTAW Reefpedia-Fish
fishbase
saltcorner
liveaquaria
marinedepot
fins
amonline
while these sites are helpful in identifying or choosing your fishy inhabitants i would still suggest asking for more localised advice in regards to their compatabilityand/or hardiness.
if i have missed any sites that people feel should be added to this list pls pm me and let me know and i will add them.
hth
a useful site that jedi has found ( thank you jedi ) is animal planet:fish guide
peteducation.com ( thanks victor bitte )