Most plumb in the sump. however, I'd skip a uv unless you're looking to cut back on film algae. they're not good for much more due to the inefficiency of water contact.
Diatoms? diatoms will go away once silicates in your water are depleted. time will help with that. a uv sterilizer may help with film algae on your glass.
This is how I hard plumbed in my Aqua UV Sterilizer which is teed off from my Vectra M1 that Includs a check valve to control the flow through my flow meter.
I mounted mine on the back of my tank.
Tank Info:
120 Mixed Reef
Ecotech M1 pump
Skimz 163 DC Montzer Series skimmer
Custom sump / Refuge
2 Maxspect Gyre XF130's Pumps
2 AI Hydra TwentySix HD LED Fixtures
Celestial C35W Maxspect Refuge Spotlight
Aqua UV Ultraviolet Sterilizer 25w
BRS Dual Media reactor
Eshoops IV Master & Slave Doser
Tunze 3155 Auto Top Off