PH down not up in DWC

Hi,
my ph has been going down instead of up lately, what could be causing it?
Or to put it differently is it normal?

thanks

I have heard that plants actually expel SOMETHING from their roots that drive the pH up... I have never heard of it going down....

Maybe it is one of the NUTRIENTS breaking down?

How big is the drop?

cheers

well it went from 5.8 to 4.9 or so they used to up the ph until a couple of weeks ago and now it's reversed

hmmm....

what do you think it could be? any changes in the SET UP?

Are you using airstones in your resevoir?

cheers

no, not many changes, apart from the fact that I am giving a stronger concentration of nutes now but ph started going down even before. I have 2 air pumps and 4 airstones, maybe since they have been there 3 months now the stones are getting clogged up and there is a lack of oxygen.
I am not really sure what could do it but I know I will have more/stronger pumps and different airstones.
well it's coming to the end so I should be fine I just wanted to know what would be the likely cause.

Are you smoking in the room or burning sulfur or anything?

cheers

not at all, the room is a guest room hardly used... why?

how would I be burning sulfur? LOL like I would spend my time lighting matches?

Sulphur burners are used to stop mold in grow rooms. I have had ph drop before it shows the plants are feeding more then they are drinking check your ppm and see if its dropping also.I would bet it is and you can bump it up at feeding time