There are two basic concerns when it comes to water quality in your backyard pond. The first is a human aesthetic: we like our water crystal clear. It's part of the beauty of a garden pond, especially when you have added fish to the mix. Fish don't care about water clarity. What they do care about is the second thing: the water's ability to support fish and plant life. It is up to us (not the fish) to see that your garden does both.

I won't detail the unpleasant facts around fish, plant, and dirt - er- effluvia. Suffice it to say that the old joke about "I don't drink water - fish *** in it!" has a lot of truth to it. If the number of fish are nicely tailored to the size of the pond then you really don't need a filtration system, although a source of moving water is always welcome. But when those fish multiply, you are either going to have to thin the school or break down and add a filtration system.
Two Types of Filtration

1. Mechanical filtration removes unwelcome solids with porous traps like brushes or sponges. These do work but require that you clean the filter as they can get gunked up. It is not unusual to have a mechanical filter with a biological component, but the primary method is the solid trapping. Pond skimmers are the most usual method of mechanical filtration.

2. Biological filtration removes toxins by using bacteria to break down excreted chemicals (ammonia) to nitrites. It sounds awful but this is a healthy process that occurs naturally in bodies of water. In the smaller garden pond we need to provide some concentrated bacteria feasting to keep fish and plants healthy, and our water clear. Biological filters host large colonies of friendly bacteria in such materials as a specially developed fiber mat. Bacteria need a constant flow of nutrients and water, so do not shut down your filter unless you mean to, such as over-wintering your pond in a snowy climate. Note that the nitrites feed aquatic plants, so if you do not keep plants you will need to do some supplemental water changes to remove the excess nitrites. A well-balanced pond stocked with healthy levels of fish and plants will only need small water changes if at all.

Algae - Good and Not-So-Good

Another thorny problem in water gardens is some types of algae growth. The velvety green algae that coats the sides of your pond is actually beneficial. It serves as an excellent food source for your fish and will not turn your entire pond green. Single cell and string algae is not so useful, and will muddy your waters. Ultraviolet sterilizers can be a good choice for single cell algae, but do not use it without the biological filter to dispense with the dead algae. There are other products to clean up string algae, which laugh in the face of the ultraviolet. As always, a natural pond balance is your friend. Please do not feel that you should rush out and buy devices and chemicals every time your water is a bit cloudy.