IMO, UV won't do much to control cyano. As was so eloquently suggested by hwarang , attack the source of the problem, which is likely nutrient levels. Water changes with nutrient-poor water, making sure you skimmer is working at full potential, good grade carbon (hydrocarbon or seachem matrix), and promoting good air/water gas exchange are all better solutions then a UV sterilizer.
But if you really want to give UV sterilizers a go, I recommend Aqua brand UV sterlizers.
If your bulbs are aging, a change may help. I find that when my MHs are going bad, cyano shows up. A change fixes it pretty quickly.
This tank has only been up since last April with established live rock from prior system.
I was using dehumidifier water for changes and top off until about November when the basement became too cold for the dehumidifier to work. Now I am stuck with filtered water (sorry not a RO/DI but an Amway filter) until I can get the dehumidifier working. The cyano has recently started.
Feeding consist of ghost shrimp and feeder guppies. No flake food and rarely frozen food.
Lighting may be my problem. I am only using a 40watt flourescent tube, power glo,. Probably junk.
This is a seahorse tank. 54 gallons, 38 gallon sump, two seahorses, 6 hermits, three snails.
Not much chance for over feeding going on. Using a training station for frozen food.