"What are Ammonia, Nitrite and Nitrates and how do they affect my pond?"

Central to the principle of filtration and fish keeping is the fact that fish are continually polluting the water they live in. These pollutants can very quickly build up to dangerous levels. A major pollutant is ammonia - which fish continually excrete. Fortunately there are naturally occurring bacteria in the pond that "feed" on ammonia and convert it to less toxic nitrite. There are also other naturally occurring bacteria that "feed" on the nitrite and convert it to harmless nitrate. So in a mature environment the ammonia rarely builds up to a dangerous level as it is quickly converted to harmless nitrate in this biological two-stage process called 'nitrification'. So far so good, however there are three problems associated with this:

These bacteria take some time (4 - 8 weeks) to colonize both the tank / pond and filter - during which time the levels of ammonia and/or nitrite can very easily reach toxic levels - a condition called New Pond Syndrome. Chemical treatment of the water can help rectify this problem as can the saving and returning of 'dirty' old water to the pond, which helps condition new tap water.

The human element! Because we like to see lots of fish in a pond, we normally overstock them. As such, this can produce more waste than the pond can naturally handle. You should therefore consider either reducing your fish population or increase your pond filtration system to handle the extra dirt.

If you don't have a filter, or have an in-efficient filter unit, these nitrates won't be removed. As such, you'll see plant growth race away and consume your pond.