pH fluctuating

I did a reservoir change last night and adjusted my pH to 5.5
This morning when i woke up my pH was 6.9

Why does this fluctuate? What can I do about this?

How big is your reservoir? How much and what kind of nutes are you using? What are you using to adjust your Ph? What are your res temps? This is all information we need to know in order to help you.

How big is your reservoir? 5 gallon

How much and what kind of nutes are you using? Foxfarm... Big Bloom, Grow Big, & Tiger Bloom

What are you using to adjust your Ph? General Hydroponics Liquid

What are your res temps? Somewhere around 60-80f

For more information, this is all documented in my hydroponic grow journal, click the hydro link below

Okay, most of your problem is that you are using dirt ferts. Hydro nutes have Ph buffers included which will greatly reduce the amount of Ph fluctuation, ferts intended for dirt do not have this because there is a medium which acts as a buffer. I grow hydro as well, and I don't have to use Ph down anymore because my nutes take care of it for me. Get the correct hydro nutes, and this will greatly help your situation.

Secondly, I have grown DWC in both 5 gallon buckets and 18 gallon tubs. It is normal for the Ph to fluctuate more so than a larger reservoir simply because of the size. Also, it is worth noting that temperature has an effect on Ph as well.

I'm just getting started in hydro myself. I did a lot of studying on it though. But that no substitute for experience for sure. It's not a very forgiving medium. But can be easy if you do it right. In my small containers (2 gal per plant) I hardly check my ph anymore. And with the smaller containers I change them once a week. I did when you were posting your problem though. I ph to low to mid 5's and checked it 3 day's after a change and it was at 6 and did not adjust it. But using hydro nutes. Not to say that I have not had my problems. I was having a bad problem that I caused myself (put mollasses in it). I was at a hydro store and was discusing it with the owner there. He was saying that plants will get used to the ph being a little high or a little low and do just fine. But they don't like any constant fluctuation. With yours you only need to buy the one you don't have, the Grow Big for hydro.