They are called flocculents. If you read up on them at the kirb, they say they could potentially do harm to your fish's gills. However, they are available at most LFS, just look for cloudy water clearers, they all use the same basic principle.
20G heavily planted tank with Eco Complete
Pressurized CO2, Eheim Ecco 2236, 1 55W and 1 36W bulb