Why does my PH always fluctuate?

Ok is it normal for my PH to go one day from 5.8 to 6.7 just over night? I'm using tap water that has a PH of 6.5 I also am using Botanicare grow, Liquid Karma and PH low to get it to 5.8 in 15 gal resivoir. I hope the info i stated helps to inform me on what is going on. Also is using tap water ok? i invested quite a few $$$ on this grow so i would like to get the full potential of my plants/buds.

are you talkin about the PH is the res? cause im noticing the same thing only i use straight vinegar to lower the ph in my 10 gal rez and after 2 or 3 days the ph slowly rises to back around 7. I dont get it either.

I consider this to be one of the flaws of a re-circulating system.

That's why I like non-recirculating Drip Irrigation systems that pump fresh nutes through drippers to your plants.

With that kind of a rez you don't need a Meter in there for constant testing. Because there is no roots eating nutrients out of the solution and changing the levels of PPM and PH.

Instead you pour fresh nutes in the non recirculating rez and they get pumped to the plants later.

Is your water well oxygenated? Do your roots look healthy?

Yes it pumps it from my res to drip on my plants. My roots look bright white and healthy. It seems everyday im always putting ph low in it just to get it where i need it. Today i bought an extra pump so it consistently circulating,and there is a air stone as well....