I think the 18watt
UV turbo twist is listed at 100gph (hey, check drfostersmith.com, they gots a great article for the accurate specs). I use a pump in my sump rated at 125
gph, that has a regulator.. I just run it at 3/4 flow and I assume all is well (I'll keep you posted if there's an ich outbreak hehe)
(the pump goes through the
uv and back to the return area of the sump...in your case, the pump hides somewheres with a tube in the tank and back up, so you might need a bigger pump to go back up to the return point, with some sort of ball valve throttle in the return to keep the flow regulated to what the
uv calls for)
EDIT -
gah..forgot to note...the turbos come with (well, mine did!) a way to horizontally 'mount' (aka hang on brackets) the unit on the back of the tank, you could divert or use a powerhead (regulating the flow!!!) through the unit back into the tank. I'd think a t with a valve of some nature might get it right through trial and error. Taking the
ph and the t and valve and tubes and all that into buckets with measured water (while being a pita) would give you a semi-accurate assesment...