As I said earlier, you cannot have too much UV on a system. The recommendations for sizes are suggestions for minimum amounts. It will not hurt bacteria that does not actually flow through it, as in bacteria that lives on or in rocks, bioballs, etc. A UV sterilizer works by exposing the water and whatever is in it (like protozoans or free-floating bacteria which usually are disease-causing) to Ultraviolet rays. This will kill most of these. The more powerful the UV rays are (like more wattage), the more effective it will be at killing those organisms. Since the 25 watt UV's are more common, they are less expensive. Why not get more power for less???
Another factor you need to know about is the speed that water flows through the chamber. The longer the contact time with the rays, the more effective it will be. You will do better to have a small pump feeding the UV, or plumb a valve with a Y-connector that will allow you to have a slower flow through the chamber.
Another thing to look for is a UV that has a quartz sleeve. It makes it much easier to change the bulb, since it has a glass sleeve the lamp slides into. If it doesn't have a sleeve, you will have to shut the system down while you take the whole thing apart, and have compression fittings that go over the lamp. And sometimes, it will leak after you get it going again. A real pain.
Sonny
