There isn't really much research into the amount needed to sterilize SW ich. That is why there is confusion. The fact we call both FW Ich and SW Ich "Ich" doesn't help even though they are completely different parasites.
So I guess what number did you settle on in hopes of sterilizing ich? From there you could calculate it.
“Use of ultraviolet (UV) sterilization to kill theronts has been suggested, based on research involving Ichthyophthirius multifiliis (freshwater “ich”). The recommended UV dose for Ichthyophthirius theronts is 100,000 μWsec/ cm2 (Hoffman 1974). However, UV doses required for Cryptocaryon irritans are anecdotal or extrapolated, and range from 280,000 μWsec/cm2 (industry numbers) to 800,000 μWsec/cm2 (Colorni and Burgess 1997).”
University of Florida, Institute of Food and Agricultural Sciences Extension outreach is a partnership between state, federal, and county governments to provide scientific knowledge and expertise to the public. The University of Florida (UF), together with Florida A&M University (FAMU)...
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So if we go with aquaculture's use of marine ich at 280,000 divide by 90,000 (Aqua UV rating) = 3.1 - that is how much we need to slow it down by. Aqua UV recommends 233 gph for 90,000... so 233 divided by 3.1 = 75.16gph.
Turnover ideally should really be 2-3 times an hour of the tanks volume but we really can't do that with a 15 watt unless it is a nano. You can get 1 turnover an hour on a 75g which would be the minimum I would go otherwise get a larger UV.
If you go by BRS/Pentairs general recommended parasite control of 180,000 then you would divide by 2 which would give you 116.5gph. This is probably more feasable for hobbiests though perhaps less then ideal.
Like I said, not a ton of research or agreement on what is exactly needed.
Please be aware that head pressure and so on will effect flow through your UV and pumps rating are often not correct so don't go by whatever the pump is rated to push. Measure it.
