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 35 gallon ressy btw) My Ph stays at a constant 5.5-5.8."
