uv sterilizers questions

Hey. I added a flow meter on the return section of my setup, but I see it constantly fluctuates (every day), which is very strange. I am wondering if it is perhaps the way I have plumbed it or if there is something wrong with my meter or hardware? I dont think its the meter as the overflow pipe...

So I just got a used 180 gallon aquarium. In great condition. Anyways, I’ve just about got it all set up but have a few questions for a uv sterilizer. I currently have a 40 gallon (no sump/canister filter) going with corals and fish and everything is going pretty good. Early on, I had issues...