Generally a cloudy tank indicates a bacterial bloom, which can be
difficult to remedy in a small aquarium. (uv sterilization would take
care of it, but it is not practicle in your size aquarium). What brand
food are you feeding? a highly digestable food makes alot of
difference, Spirilina algae based foods are great, Sanyu and Hikari
make excellent ones too. Avoid Tetrafin. A ph of 7.5 is important, so
is your carbonate hardness. Sponge filters are an excellent choice for
bio filtration, especially in small aquariums, but they do not work
over night. You need to allow time for them to seed, and to never clean
them in tap water. Here is a blog about them:
http://spongefilter.blogspot.com/. Wonder shells (non-medicated) can
help with carbonates and ph. I am sure others can add more.
Carl
http://www.americanaquariumproducts.com/