Wildchick, are you using rockwool?? Because rockwool is notorious for increasing the pH. It is highly recommended that you soak your rockwool in a pH 5 solution (not lower than 5 as the rockwool will breakdown). Even after soaking your rockwool it still increases the pH of your nutrient solution, however, it not as bad. Grodan has now placed labels onto most of the rockwool packs informing customers how to do this.

Also try and select a nutrient with a pH buffer in it. The GH nutrients have no buffers but the Canna Aqua nutrients do. It is recommended that you mix your nutrients (Canna Aqua) and let it stand for one hour before adjusting the pH. This gives the nutrient's pH buffers time to act, this will help keep the pH stable.
You should also be changing your reservoir every week...minimum . You never know what your plants are doing...they may be very thirsty and take in more water than nutrients...resulting in your reservoir having a huge concentration of nutrients. Or the exact opposite might happen...they may be taking in nutrients but not too much water. This is going to result in your reservoir having a very weak nutrient mix. This is more than likely the problem with your pH fluctuating so much.
Hope this info.is of some use to you:thumbsup: Remember..."HAPPY PEOPLE make HAPPY plants":stoned: