water ph low, need help

i'm new at this hydro thing and just setup my first grow in it. the ladies are currently just poking out with their first set of leaves. i check the ph about once a day and for some reason the ph keeps dropping low on me and i keep having to add ph up everyday, and i can't figure out why. some help on this would be appreciated. here are the details of my system.

flood and drain system
20 gal reservoir
using flora grow 3 part nutrients, seedling and cutting concentrations
distilled water
rockwool grow cubes
clay pellets in tray
currently have the pump set to come on for 10 min 4 times a day

thanks.

i see you are using rock wool - that may be effecting your pH. i use the G.H. three-part and it does buffer and stabilize pH. you must be spending a mint on distilled water for a hydro grow!

so are you saying that eventually it will stabilize to the proper ph? how does that work exactly since something has to change the ph, it can't just change on its own. something gets added to or removed from the water that changes the ph. also, distilled water is only $0.79/gallon, so it's not expensive

the nutes buffer the pH - when are you checking pH? do it after adding nutes and not more than once a day until you get it stabilized. also - don't rule out a crappy/faulty pH meter (or just old batteries or maybe needs calibrated).

so yes, pH will change "on its own" depending on the nutes and how much water/nutes the plants are taking up. did you soak your rockwool before you started using it?

i mentioned the priciness of the distilled water cause of my own water use experience - i change my 20 Gallon reservoir every week and have to add 2-5 gallons to it everyday - that would add up pretty quick over a three month grow but your mileage may vary.

Here's a link with more info on pH than you may ever want to know ...
CC Hydro 101 by Earl - Cannabis Culture Forums