I am in no way an expert, but I have done a lot, and I mean a lot, of research on UV's as of lately after an unknown parasite slowly wiped out my entire tank, but that is a different thread. So I am now in the process of setting up a UV Sterilizer on my 180 gal before I start restocking. This is what I have learned after my research:
Purposes for using uv
- water clarity (higher flow)
- kill parasites (low flow)
First, Uv will kill (or damage them enough so they can not reproduce) only the things that go through the unit. Beneficial copepods and amphipods pods, which are benthic, (bottom-dwelling organisms often attached to hard surfaces such as rock, sand bed and glass), will not go through the unit and will not be harmed. However a few may get sucking in the uv. Only the free swimming ones will go through the uv. So no worries about killing off all your entire pod population. No, it will not completely eliminate all ich and other parasites from your tank but you shouldn't notice anything on your fish or have a major outbreaks that can wipe out your tank if using a properly sized uv.
Second is size of the unit. Most UV's are grossly understating the correct size you need to actually
kill things. In this case going bigger is better! After my research and talking to local hobbyist that use uv, I am going with a 120 watt unit from Aqua-UV (formerly Pentair) for my 180 gal fowlr tank. If you are looking for water clarity then that is a different story and the smaller units will be fine.
Third is the flow. For water clarity faster flow is fine, but for killing parasites you will need slower flow, use the manufactures suggested flow rate for the size of the unit.
Last is placement. To be effective, 100% of the water needs to move through the uv unit. If you have it plumbed going only through your sump this will not be effeictive. Therefore placement is important. The chart
@scardall linked in post #35 is a good example how to properly install a UV.
As for me, I will now run a uv 24/7.