Not to intentionally take the other side of the fence, you need to consider how large the UV
light is and water flow. If your using a very slow water flow then its not unreasonable to suspect the temp over time would rise a few degrees. I suggest a very simple, unscientific approach of turning it off for a week and see if helps. One week should not cause you any problems. As for running it part time thats perfectly acceptable. A timer can help with that. Most folks with a UV
sterilizer don't read the dosing recommendations anyway and end up overexposing a tank..But I digress. Please do try shutting it down at least for a couple days and see if it helps.