PH water supply in container rises too high after one day adding PH down

[Robogreen]

I usa couple gals comes out of tap at 7.8 ph. I add PH down to 6.5ph and 2ml cal/ mag per gal . for soil use. Next day Ph jumps up to >7.1
Never had this problem using RO water and nutes in the past for coco. Never jump this hi this fast.
Any ideas why so fast PH jumps up? Water is little hard out of faucet (400ppm) comes from lake mead.
Thanks all

 

[cannabineer]

I usa couple gals comes out of tap at 7.8 ph. I add PH down to 6.5ph and 2ml cal/ mag per gal . for soil use. Next day Ph jumps up to >7.1
Never had this problem using RO water and nutes in the past for coco. Never jump this hi this fast.
Any ideas why so fast PH jumps up? Water is little hard out of faucet (400ppm) comes from lake mead.
Thanks all

You have bicarbonate hardness.

Your initial pH includes freshly generated carbonic acid, which is what gives seltzer that delightful sour zing.
But just as seltzer goes flat, the carbonic acid goes away.
You have just experienced the bounce.

If you adjust pH now, it should be stable, or nearly so.

 

[Robogreen]

makes sense. Thanks