Biological filtration which is the oxidation of ammonia to nitrates by bacteria which colonize in these filtration materials. These are called nitrifying bacteria. Biological filtration is extremely important since it keeps ammonia and nitrates from accumulating to harm your fish. Chemical Filtration usually refers to filtration using activated carbon ammonia absorbents or water softeners. It is suitable for small tanks.
Biological filtration is possible if proper mediums are used such as foam or a gravel stock.
It is not optimum for biological filtration as there are problems rinsing and changing filter materials properly. Digging fish can negate filtration and live plants may not prosper. The pH may drop rapidly if water is buffered poorly. On larger aquariums, used alone, chemical filtration is not possible. Foam filters function both mechanically and biologically.