I have been doing this for years. Get a medium sized derasa (8-10") per 50g or so will do a great job in nitrate and phosphate reduction, of course depending on your bioload. I personally have a 10" deresa, a 4" maxima and 5" crocea in my 120 with 3 fish and have no detectable nitrates or phosphates. Something to think about for those of you fighting nitrate / phosphate issues. Also here is an interesting article about it
Clam Biofilter
Clam Biofilter



