The answer is slightly deeper than that. We have to go over ideal kill rate for the sterilizer for marine protists as most are concerned with ich for sterilization purposes. For clarification, the 25 watt would work all day.
Perhaps I could point you to another thread I contributed as today is a busy day at work and I don't have enough time to explain it as clearly as I would hope to.
Here is an except I copied from it and also the link to the thread. I was responding to a reefer about his current setup. Please also keep in mind that in my own experiences, when I hooked up my UV to my manifold in my sump, I had marginal results. Its only when I hooked it up to pull water directly from the lower portions of the display, fed it into the sterilizer, and back into the display, did I have the best success. My theory is the free swimming trophonts tend to stay towards the lower portions of the water column and most don't get high enough to spill over the overflow and into the sump to be irradiated. So having your UV in the sump hooked up to your manifold is almost useless IMO. This was backed up with me being able to keep my powder blue tang in my sump ich free for months but the minute I moved it into the display above, it got ich immediately and succumbed to it. The UV in the sump was effective to keep him free of ich but when I moved him to the DT, it was ineffective. Hope the info helps.
https://www.reef2reef.com/threads/uv-overkill-or-nay.245079/
"You have a 150-watt HO Emperor you plan to use on a 300-gallon total water volume tank. Emperor suggests for killing marine protists (ich), you would need an exposure of 180,000 µWsec/cm2 running at 840 GPH to do that.
According to research by the University of Florida:
https://edis.ifas.ufl.edu/fa164
"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)."
So the extrapolated saltwater ich exposure needed to kill it with UV is anywhere from 280,000 to 800,000 µWsec/cm2. I've run with 336,000 µWsec/cm2 because I pulled that from a chart I saw many years ago for an exposure level to kill marine ich.
So to adjust Emperor's GPH from the 840 GPH needed for 180,000 µWsec/cm2 exposure to almost double the exposure suggested at 336,000 µWsec/cm2:
336,000 µWsec/cm2 ÷ 180,000 µWsec/cm2 = 1.87
840 GPH ÷ 1.87 = ~450 GPH
Running your UV at 450 GPH will effectively kill marine ich protists going through your 150-watt UV unit. The problem is most recommend at least a 3x turnover per hour for UV to be effective. Basically, the 450 GPH will kill any ich that goes through your UV, but if you don't have it turn over the tank volume at least 3x/hour, you're not killing enough of them to make a true impact to the population. For 3x turnover of your 300-gallon system, you need at least 900 GPH. So effectively, believe it or not, your unit is actually half the size it needs to be for your system as you're only running 450 GPH through it.
Again, these aren't hard rules of thumb. The 3x turnover is not a hard rule, just a suggested rule. The 280,000 µWsec/cm2 - 800,000 µWsec/cm2 extrapolated marine ich exposure value is not a hard line either, especially since it's just extrapolated from a freshwater ich exposure dose.
I just wanted to do the calculation for you to let you know where you stand and to give you comparison to suggested industry rules-of-thumb. Your own application and your intended purpose will help you decide what you really need to do. Just trying to help out a fellow hobbyist."
