I run an Emperor Aquatics Smart UV25 in my setup here. I have mostly SPS and LPS and a very high bio-load. The UV has been up for over a year now.
This is how I setup mine:
Overflow --> T into external skimmer and into sump
Skimmer --> T off into sump and UV
What is important is you need to control the GPH through your UV. You can do that by putting a ball valve inline between the Skimmer T out and the Sump. The backflow will travel through the UV. Mine is set @ 50GPH max. Which kills almost anything...
You are suppose to keep the glass sleeve inside the UV clean to alllow better contact. So if you don't run some sort of filter before the UV, you would have to clean the glass sleeve more often.
Also I do run Carbon and PhosBan (both changed bi-weekly) @ the last chamber in the sump before my return pump.
The Turbo Twist 9W model requires 125-150gph, and Lunner is correct, it's best to use a prefilter on the water entering the UV or in a short time it will be ineffective.
The unit is not submersible, so it must hang outside of the tank or sump.
But it does come with the bracket with suction cups to attach it to the outside of the tank
..then again some will look at it as yet another potential place for a water failure, especially if you have your overflow feeding it...
IMO they really aren't a necessity on a reef tank as they don't discriminate what bacteria they are killing.
If the thought is to use it to avoid issues like ich, I don't know that the DT is the proper place to be running a UV as any ich trophants that drop off a fish aren't likely to be free floating in the tank but will drop down to the substrate where the UV will have no affect.
Maybe something great to keep around and use on a QT tank? There it's affect would be great as QT's don't have a sandbed and the water motion would keep most of the nasties suspended in the QT allowing the UV to come in contact with them
..can you tell I'm not a fan of them in a DT? lol For me they give a false sense of security that they don't deserve...
If you want to use it to get rid of algae that is forming on your glass, you are treating the symptom and not the cause
I know I recently read on here that the Elos Phosphate kit is pretty accurate when compared to the readings from a Hanna meter, but would have to say if it's isn't an Elso kit you are using for that, your info may be inaccurate unfortunately.
Also keep in mind that just because a test kits tells you your numbers are low, it may well be because the algae has already absorbed the nutrients and your test shows a low number but it doesnt mean they aren't in the tank in another form...like the algea on your glass.
All, I run my UV for 4 hours a day and have certainly seen healthier fish/corals with no impact on fauna/inverts etc. Water seems more clear, healthier and corals have great PE. Running additional line from sump to UV to tank.
It's also a Coral life 9w .. will always use product going forward .. Garf also likes to use them on their tanks and swear by them.