can I use wet and dry filters for fresh water aquariums?

Answer 1

Yes you can. These are more commly found in saltwater tanks (where the fish are more sensitive to nitrogenous wastes), but they still work in freshwater. They'll give you added water volume to dilute wastes and added surface for bacteria to colonize to remove ammonia and nitrite - both features are a plus if you have fish that produce a lot of waste.

You don't need to buy a super-expensive setup, either. Just do an internet search for DIY trickle filters and you'll get lots of designs/methods to make these yourself like this one: http://www.aquaden.com/phpBB2/articles2.php?type=diytrickfilter

Answer 2

You can use a wet/dry on a freshwater. I have a 55 gal freshwater and I use one. These are nice because they hold more water and are easier to clean the filter. Here is the link to the one I have.
http://www.drsfostersmith.com/product/prod_display.cfm?c=3578+3579+13648&pcatid=13648