I use a 25 watt on a total system volume of about 120 gallons. Mine is a Aqua ultraviolet sterilizer but emperor aquatics is another great name. I would say anything in th 25 to 40 watt range would suffice.
I have an Emperor 40W on my 120. I like it.
I've had my clowns get little red fungus growths in their mouths occassionally, but nothing serious. Definitely no parasites. I use it more for zapping bacteria...and keeping my water clear.
smeese-UVs zap bacteria, protozoa, and algae given the proper exposure time (dependent on flow through the unit). because of the high energy they also create small amount of ozone which also has bacteriacidal/algacidal properties-and maybe some other water clarifying effects.
if anyone is interested i have a brand new emperor aquatics smartlite UV-18W-i got it from devin but i never had the opportunity to hook it up-now i want a larger model for a small pond i'm hoping to build. 1/2 off the price i paid devin (don't think of it as me trying to steal his business-cuz i'll be
buying my next (and more expensive) one from him too).
I think the usefulness depends on two things.
1) the kind of system you have. If you just have one tank with lots of sand and rock where parasites can go through their cycle. It can be hard to get them out to the UV to get zapped. If you have a multi tank system, or do not use sub-strait. Certainly this can eliminate spread of... well life The flaw in this would be if parasites spend a time not attached to anything at all.. then they would statistically nearly all eliminated from the water column in a few days.
2) This one I am sure about. Get the right slow flow through the unit. Look at Devin's site. There will be numbers on the strength of the flow and what it will kill. If your return flow is faster than this, set up a closed loop or some other method with a smaller pump. If you push things through the UV too fast, it is pointless.