pH rising like crazy! First ever DWC. Help!

Hi everyone,

after 10 years growing in soil, i went for a diy DWC system.
Seedlings went into the netpots in rockwool, 2 days ago.
I use tapwater.
EC of the solution was .71 on day 1, .67 on day 2 and .66 today.
I was expecting the pH to fluctuate a bit and that’s why I have been measuring it daily, but the results are shocking. It went from 5.8 to 7.5 in 24h. Used pH-down to about 5.5 and today it was at 7.5 again. Used pH down to 5.4 and within one hour it has gone to 6.5 again.

Temps are dialed in, I even use an AC to vent cool air to my reservoir.

What am I doing wrong?
I ve read that in some cases the rockwool cubes tend to raise the pH, could that be the case?
Any tips are greatly appreciated.
This is my setup:

Tap water as in hard water?
I think R/O is preferred

It's likely do to a type of cyanobacteria in your reservoir which is usually caused by the reservoir getting too warm. Maybe turn the ac down a few degrees lower and see if it helps. Is the water turning cloudy at all, or are the roots turning yellow/orange?

Also in dwc I'd recommend going full synthetic with the nutes. From my experience if anything organic gets in the res it's likely to cause problems. So if a leaf or a fly or something falls in your res make sure to fish it out.

you aren't planning on growing 4 plants in one bucket are you?

"I use tap water"

right out of the tap?

filter it to at least 0.50 micron. made huge difference for me in ph stability.
what would fluctuate from 5.75 to 7+ overnight with 1.0 micron water filtration...
now doesnt move 0.25 in 24 hours with 0.50 micron filter.
nothing else changed.

clear airlines are a no-no too.

and trying to stabilize pH of only a few gallons is next to impossible

rkymtnman said:

you aren't planning on growing 4 plants in one bucket are you?

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I‘m not. They will be there for 2 weeks then they get a reservoir each.
What do you mean „clear airlines are a no no“?

giannoulis said:

What do you mean „clear airlines are a no no“?

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they will allow light into your res which will lead to algae and/or worse (root rot)

and i prefer rapid rooters vs rockwool. they are more forgiving to wet/dry conditions i think.

rkymtnman said:

they will allow light into your res which will lead to algae and/or worse (root rot)

and i prefer rapid rooters vs rockwool. they are more forgiving to wet/dry conditions i think.

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100% on the rapid rooters. They're fool proof.

rkymtnman said:

they will allow light into your res which will lead to algae and/or worse (root rot)

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I'd replace those two "will(s)" with "can" and "could".

I've used clear air lines for a couple of years with zero drama. In my current grow thosr clear airlines are just going over the lip of the bucket with top plate just sitting on it leaving the side contantly ajar. 100x more light is getting in through that gap than a simple clear air line and I'm 100% fine. And Im not running a sterile system. I use tap water straight out of the tap (no letting it sit or filtering it). And I did only 1 res change (when changing to flowering nutes) throught the entire grow, just topping up every day. My 4x4 is 5 weeks into flower and completely full of 4 plants that are as healthy as can be. No algae, no root rot, no bad smells..nothing.

trippsy25 said:

And Im not running a sterile system. I use tap water straight out of the tap

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if you are on muni water, then you are running sterile via chlorine or chloramine

I use tap water. EC is 150, and PH is 6.8. I then lower it to 5.5 and pump it in the DWC reservoir.
After a day, it goes up to 5.9, 6.. and it stays there.

I found that it doesn't mix well for some reason. I get stable reading hours after I mix it, but not immediately.
So in my case, I always aim for 5.5, wait about 20-30 minutes, test again, and then add to reservoir, and I test the next day again.

Are you using a citric acid based PH down by chance? Mine did exactly the same after changing from top fed recirc to RDWC. I switched to phosphoric acid based PH instead of citric and the problems all went away

bubblescrogs said:

Are you using a citric acid based PH down by chance? Mine did exactly the same after changing from top fed recirc to RDWC. I switched to phosphoric acid based PH instead of citric and the problems all went away

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Nope, i m using phosphoric acid too