Newbie pH problems

Hey everyone. I have a quick question about my bubbleponics set up.. I started the pH in my reservoir at 5.5. It moves up during the day to around 7.5. I switch a gallon of 7.5 water out with 5.5 but by the end of every day its back up to 7.5 or higher! Please help. This is not where I want my pH to be and I need some assistance. Can someone please tell me how to keep my 6 gallons of hydro water to stay at pH 5.8-6.0 without having to adjust it twice a day?

I am a total noob at soil, never even tried hydro. Anyhow, the thought occurred to me: Does PH change according to water temps at all? Well, not so much as the true PH, but maybe the meter doesnt accommodate for temps? Have you tried using another meter to verify your results? Like I said, just a noob, but thats my 2 cents. As for me, Im sticking with soil, I sleep better at night knowing Its harder to screw up!

Perhaps a little more info as to the setup and you'd get better help. Perhaps the nutes you have are time releasing? Maybe something is evaporating out of your water during the day? Most of the things ive read say ph goes lower over time, so Im curious as to what you find. Keep us posted!!