John Rutz sez:
Ok so i should be able to get this from the catalog website but they
dont seem to address this or Im missing it

how oftern should the pond volume go thru the UV? or how many gph
should the thing be capable of based on my pond being 8000+ gallons?

the ones I have found that can take my pumps are in the 5000-8000 dollar
range OMG

First off, you want slower water flow than you think. The light needs time
to kill things in the water. So you will need to divert some flow.

Second, the manufacturers like to supersize to higher profits, and ponders
tend towards the "nothing is too good for my pond" philosophy, but that
misses the point of a UV. The UV kills things. That's what it does.
Things have to reproduce faster than killed to cause a problem.

There are two conditions you can use your UV for: clarified and sterilized.
Clarified means, "no green water". Sterilized means, "no green water and a
lot fewer other things". Sterilized is much more thorough than clarified,
and doesn't do any harm, but it may not do much more good either, unless
you have a somewhat infected pond.

To clarify, the only requirement is that stuff get killed faster than it
blooms. It takes a while (days) for green water to "show up", so it
doesn't take much to prevent it. The goal is more to kill off enough that
it isn't visible than to render the water unnatural.

The difference is in cost. My pond (see pics at
http://www.fringeweb.com/Ponds/kc/MyPond.html ) uses a maybe-200gph pump on
a 15 watt frog. My pond is nowhere near yours in size, but you can tell
from the pics that it's not tiny, and it stays crystal clear.