UV sterilizers, though they do not sterilize, are essential to keep water free of suspended algae.

UV sterilizers must be installed after a mechanical filter or else the quartz tube that encloses the bulb, will be coated with dirt in a day or two preventing the UV rays from reaching water. If you have hard water or keeping the GH (general hardness) high, then once in a while you must clean the quartz tube with CLR or some other cleaning agent.

UV sterilizers are known to leak and sometimes implode when the pump is stopped. Stainless steel sterilizers do not seem to have these problems. Some have connectors that fluoresce in the dark when UV is working (Matala, GCTEK). This is a nice feature since you want to know as soon as the UV stops working.

We recommend every pond using 2 different pump circuits so the pond can continue to function even when one pump fails. Similarly, having 2 UV sterilizers connected to two different pumps is a good idea. Often you must buy a UV that has twice the capacity of the manufacturer claims. But if you have 2 UV sterilizers, you can buy the UV that is recommended for your pond volume. Remember to change UV bulbs at different times so both do not fail at the same time.

Some manufacturers have wipers and other features. These do not enhance performance of a UV sterilizer. What is really important, is the amount of passing through the UV sterilizer, performance of the UV bulb and cleanliness of the quartz tube. For example, if a UV sterilizer turns over the pond volume in 5 hours, it will not be effective. Algae multiplies every 20 minutes. UV sterilizer will not be able to keep up with algae growth.

Most important thing is to check for the type of replacement bulbs. GCTEK replacement UV bulbs are only available at their dealers. Naturally, they are quite expensive and do not arrive quickly when you need them. Besides, the design of their UV sterilizer is so poor that you may break the quartz tube while replacing the bulb, costing you another hundred bucks. Matala on the other hand, uses regular Philipps UV bulbs and can be replaced without breaking the quartz tube.

Some people may believe that fish need algae for growth. Pond walls and floor will have plenty of algae to support the zooplankton that the fish love. Suspended algae robs oxygen from water, causes pH swings. You can control algae without a UV sterilizer. But it needs lot of effort.

You should grow plants hydroponically (use styrofoam with cutouts and place regular pots with plants that love moisture like impatiens, hydrangea, heuchera, taro, caladium). They consume nitrate and phosphate (that are essential for algae growth). They also prevent sunlight from reaching water. Fish will love the zooplankton that grow on the pots, Styrofoam and get to prune the plant roots.