Your PH swings are a huge problem bro.
This is what to do.
Now- how to control your PH...
There are a couple factors that cause your PH to drift. The most common is running the nute solution too strong or too weak. If it's too strong, your PH drops. If it's too weak the PH rises. You have to find the balance point. If you find that your nute recipe is say, a little strong, and say you have a 20 gallon ressy, mix the nutes for 18 gallons. If it's a little weak, mix it for 22 gallons. You follow?
The other common mistake is mixing your nutrient solution and trying to adjust the PH right away. You have to allow time for the ph buffers in the nutrient solution to do their job. Mix the nutes up, and bubble or circulate for 12 hours before even looking at your PH. You will find the PH to be almost right on target.
I use little or no ph adjusters. (Except during flush). I use tap water, with a starting PPM of 300-400 and a starting PH of 7.4-9.0 (Thats crazy you say! The water is too hard, you will never get stable PH and you will have too much Ca, causing a lockout of Mg!) Bollocks I say.
Every week, on Friday I drain the ressy and fill with plain water. I let that run overnight giving the plants a little flush. This also allows the chlorine and ammonia to evap. On Saturday morning, I mix the nutes into the ressy. I DO NOT adjust ph at this time. Let that run as is overnight. On Sunday morning, I check ph and adjust if needed. Usually don't have to.
Doing things this way has saved me from constant PH drifts, and using about 150 ml of ph down every ressy change, and a little here and there throughout the week to keep it in the desired range. Now, the most I ever have to use is about 30 ml.if any at all. (I have a 100 gallon ressy btw) My Ph stays at a constant 5.5-5.8.
Another thing to keep in mind...
Your ph can flux strongly depending on your system so its important to get that dialed in along with what your strain likes and can handle..
Its not good to just keep dumping in chemicals to maintain your ph at 5.8 let it drift as low as 5.2 - 6.0 that way your plants can get everything available to it. When I see my shit fluxuate low then high I know my ladies are absorbing nutrients properly. Make sure your nutrient temp is around 68-72 deg this is important as well.. Especially if you are doing DWC At 78 deg you will get pythium.
GTD