Advice on pH balance

A question from a fellow grower:

I wonder if I can bother you for some advice please. I am growing using a hydro system in pebbles.

I seem to be having alot of issues with my Ph balance. I just changed my water solution yesterday. The water is Ph 7.5, I added my nutients which took the ph up to 8.6, then I used Ph Down and took it to 5.5 ph. When I checked it this morning the Ph seems to has shot up to 16ph (have added ph down but it just wont come down) I have got it down to 7.1 but it seems to be stuck around there.

I am in week 4 of flowering and things have been good well so far.

Seems like you might need to change your water.