Reefer 300 display water level increasing at night

[hers0109]

I have a Red Sea Reefer 300 that has been set up for about 8 months. In the last few months I setup an AquaUV sterilizer in line with my return pump. I turn on the UV at night 8pm-8am though water flows through it 24x7 because it’s set up in line with return pump. Plumbing pic attached - using flex tubing.

My water level during the day will stay consistent but at night the level in my display and overflow will increase a few inches. Once UV turns off the water level slowly goes back down to daytime level.

Doesn’t seem like this should happen… is there a way to fix this without just running UV 24x7?

 

[Lavey29]

Probably need to adjust pump flow to compensate for the increased head pressure when the UV comes on. Not sure why exactly because it's flowing through it when the UV is off but maybe the dwell time increases when the UV is on.