Thanks guys. Viper I am running an MV controlloer set at 400. Couple of months ago I suddenly got White Spot and I put up a 11w UV which I run permanently and I also got the Ozone. When I started the MV probe read 48. I set the controller to 450 and the ozoniser ran constantly for about a week to get to 450, i then adjusted it to 400. Now I notice that in the day with all the lights on it drops in the afternoon to about 192 and during the night it gos up again. The unit only runs until the reading gets back to 400.Are you running your ozone through a controller?