The Nitrification Process takes up a whole lot of oxygen to perform, if you want to optimize your filters performance, you must aerate it as much as you can. Without an oxygen rich enviroment, you filters perfomance will be stunted and will not be efficient, you also want the biological filter not to be dirty, because the bio will work on the solids before the chemicals such as ammonia; Nitrate; and Nitrite. You should add Calcium Carbonate or some other Water Hardening Substance, because this is like food to the biological matter in you filter, keep a good kh and gh in your pond this will allow the biofilter to develop correctly.