I doubt the UV unit added that much temp. I would suggest it might have to do more with the humid weather we have had the last few weeks. Have you had your windows open or running AC. Humidity is usually the largest factor in tank temp. this time of year. 84 is about on the upper acceptable range. If it gets much above that I would be concerned. I try to keep my temp. in the 78-82 range.

Are you running cooling fans on your tank. If not, short of a chiller that's the best thing you can do. If you are running fans then you can replace them with higher CFM fans. That's what I did last year to my tank. I have spare fans I only run when I am having problems. They only get used occasionally. I had to run them quite a bit in the last few weeks because the windows have been open and we have had this overcast humid weather. Spring and Fall are usually the time of year we have temp problems, simply because of the humidity. During the summer, we have the windows closed and the AC unit will keep the humidity much lower in the house.

Remember that the tank primarily cools itself through evaporation. Tanks with a glass cover have a lot of problems this time of year. Less evaporation means less cooling. Less cooling means higher temps.

I hope that helps.