Q. I live in sunny Hawaii and my fishpond is so green with algae that I am thinking of quitting this hobby. It seems like everyone here has the same problem. In previous articles you have suggested that a 30-watt ultraviolet sterilizer will eliminate algae. Do these devices really work? I have a 1000-gallon koi pond. What size sterilizer would I need? What capacity pump should I use?
A. I guarantee that an UV sterilizer will clear your pond. They are 100 percent effective, assuming you match the wattage and flow rate properly. There is absolutely no reason that anyone should have to put up with an opaque green pond. Moreover, you will find that your mechanical filter will clog less often once the UV sterilizer has been on for a few weeks. This is due to the elimination of planktonic algae.
A single 30-watt sterilizer should be sufficient for your pond. Make sure that you buy an “inline” sterilizer where the water flows into the unit and around the bulb. For outdoor pond installation a quartz sleeve is an absolute must.
You should get a pump that is capable of circulating 500 gallons an hour through the system. Depending on the distances involved, the pipe size and the geometry of your pond, this may mean a pump with a 1000-gallon-per-hour capacity. The important thing is that you maintain the suggested flow rate through the UV sterilizer. Either a submersible or non-submersible pump will work fine. You will notice the pond clearing within a week.