The purpose of a filtration system in a closed system such as a pond is to emulate as closely as possible the natural nitrogen cycle of nature. Fish eat food and create waste. Debris falls into the pond and decomposes. These dissolved waste products, primarily ammonia , meet nitrifying bacteria in the biofilter and are digested, creating their own waste products.
Nitrates are far less toxic to the fish population than the original ammonia waste product.
In the natural world , a myriad of other bacteria and countless other microbes would digest this nitrate waste product and therefore consume this nitrogenous waste. Plants, also use nitrates as a nutrient source. When your fish are active, it is best to feed them twice per day - once in the morning and once in the late afternoon or early evening. A change in their eating habits is often the first sign that something is wrong, so knowing these details about your Koi and Pond Fish will allow you to head off problems. By all means, treat your fish to an occasional snack of leaf lettuce, celery greens or other approved fresh tidbits, like brine shrimp or tubifex worms.