What about a Review type database where Local fish stores as well as online retailers can be reviewed. Would be easy then for someone to find other fish stores in their area as well as good online retailers. In that you can find a way to denote site sponsors, make them them stand out from the others a little. If you wanted to you could go a step further and review items like different tanks, filters, etc.
Not sure how much use it would get but what about an article database? Not just a sticky but have them turned into actual articles in a separate article section of the site? All in one place and easily accessible. DIYs, Cycling, Chemisty, Compatibility, etc.
Calculators. Not sure what it would take to get something like this but could come in very hand. Substrate calculators, water volume, unit conversions....etc.
Well just a few more suggestions to add to the list!