PH won't stop dropping

New to forums and still learning how to do this so if I’m doing something wrong please advise. From what I’ve read I’m supposed to make a ticket for my question so here goes-
I haven’t grown in about 4 yrs but since we’re confined to our homes for the foreseeable future I decided to start it up again. My grow consists of 3 aeroflo 36 systems (1 in veg, 2 in bloom) and an ez cloner (which I will use after I find the right pheno from this batch of 20 seeds)
My problem is the ph keeps dropping along with the ec. I’ve done water change after water change using h2o2 to rinse between res changes (because i was worried about root issues) Here’s how it’s been going, I changed the water in the reservoir last night and the ph was 5.65 and 24 hrs later it’s down to 5.5. I know that’s not much but it’s consistantly been doing this since I put them into the aeroflo system, sometimes it swings a little more than others. I know a dropping ph can mean root problems (hence the h2o2) but my roots are bright and beautiful so I really hope that’s not the problem. Everything else I’ve read suggests that I’m pumping too much co2 into the reservoir through my air pump/stone. I’ve never experienced this before in 5 yrs of growing and my set up is exactly the same, the only thing that’s changed is the nutrients and the strain. (I’m using the full line of CX nutrients and growing gg#4 from feminized i49 seeds this round which could be a whole other topic as I’m not very impressed with either at the moment) I just unplugged my air pump so we’ll see if that helps but in the meantime does anyone have any other ideas/thoughts on this?

  • What strain, Seed bank, or bag seed
    i49 seeds gg#4 feminized

  • Method: Soil w/salt, Organic soil, Hydroponics, Aquaponics, KNF
    Hydroponics, general hydroponics aeroflo 36

  • Vessels: Pots, Grow beds, Buckets, Troths
    4 ft grow chambers/troths

  • PH of Water, Solution, runoff (if Applicable)
    Started at 5.65 after reservoir change, 24 hrs later it’s 5.5

  • PPM/TDS or EC of nutrient solution if applicable
    EC 1.30

  • Indoor or Outdoor

  • Light system
    2 - 1000 watt metal halides & 1 bestva 3000 led ON 24/7

  • Temps; Day, Night
    Between 75-80 constantly (lights are on 24 hrs)

  • Humidity; Day, Night

  • Ventilation system; Yes, No, Size
    None right now because it’s still cold enough outside to keep temps in the recommended range

  • AC, Humidifier, De-humidifier,
    Humidifier & small space heater

  • Co2; Yes, No

Thank you in advance!


I don’t think it should make any difference in the ph dropping or not but my starting ph out of my ro is 5.5.

RO should be 7.0 not 5.5.


Dropping pH usually is a root issue or other microbial activity. I was using GH Floralicious and saw the pH drop like this. I change water out once every week. If you are keeping up with that then you may need to get ph up with a PH Up like General Hydroponics and press on. I found once I got it under control around 5.9 is would stay there. I had to use a fair amount of ph up to get there.

I’ve been changing the reservoir every 5 or 6 days to try and figure out what’s happening. I just calibrated both of my ph/ec monitors with fresh solution (1 bluelab guardian and 1 hanna groline monitor) and it seems all they can agree on is the temp and the ph of the calibration solutions. They both read different for my tap water, my ro water and my reservoir so idk what the actual ph is but I can take both readings and figure out that I’m starting in the proper range and dropping out of it within a cpl days. I just added gh Ph up to the res and it took much more than I was comfortable using to get it from 5.1 to 6.0. I would like to raise it to 6.5 since its dropping but that would take 1/4 of the quart, I’ve never needed more than a cpl cap fulls before. This is my 1st run in 4 yrs and nothing seems to be going the way I remember it, I hope I just got some bad seeds! There are a couple different phenos and 1 of them has been a b!tch so far, nothing I do makes them happy, the other pheno is coming along much nicer but only one out of the whole batch has been lush and green the whole time…I can’t wait to clone her!

Can you post a picture of your set up :v:


Is it possible that any water lines have junk in them from sitting until now ?

Do you have a pic of the system as well ?
Would be nice to see exactly what the setup is.

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I cleaned all my water lines, my chiller, my manifold and my laser lines in my grow chambers w a really heavy dose of pyur (35% peroxide) as I always do between runs and I also ran it through the full system before I put the plants in. Everything appeared to be clean and sanitized, I’m pretty OCD about cleaning.


@BigJ This looks similar to the one that you are trying to set up


I ended up using a lot of ph up to get it up to 6.2 (on the hanna) and 6.5 (bluelab) and 12 hrs later they read 5.9 and 6.1. I also unplugged my air pump just to see but that didn’t stop it from dropping. They also haven’t drank any water since I adjusted the ph and they were just starting to grow again after stalling for a few days. I’m thinking about getting a starter kit of some other nutes and trying them for a wk to see how the plants and the ph react. I’ve changed the water 5 times (6 total reservoirs) in 4 wks trying an EC of .5 all the way to 1.8 and nothing has been “right”

How old is your R/O system?

Any reading gotten from 10 ppm or less water is going to be unreliable. There are no solids to hold it to a specific PH and any reading you get will be ‘spurious’. This is why I don’t bother with checking and adjusting PH when watering with R/O.

@Chilla what is your rez capacity? What is the temp of rez? What CO2 are you using? How old are your plants?

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I will do some checking, but not sure lack of solids causes you to not read acidity or alkaline state of a liquid.
I read stable ph for RO and it predictable changes with additives. Might be I have just done it so often I know what to expect.
To be fair, I never water with plain RO either.

By definition there is no potential hydrogen in pure water. You simply can’t PH pure water with any instrument and obtain an accurate reading.

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Interesting. I am still a little confused since water is H2O. It’s not that I doubt what you are saying, I just don’t know why you can’t read hydrogen if acidic hydrogen ions (H+) in low PPM.

Pulled this from NC State U

“By definition, pH is the negative log of the hydrogen ion concentration of a substrate solution or a measure of acidity or basicity of a solution. The more hydrogen ions within the solution, the lower (more acidic) the pH value will be and vice versa. How does a pH meter accurately provide you with the value? Modern meters have a single probe which contains two types of electrodes. One
electrode (measurement cell) determines the hydrogen ion activity, while the reference electrode offers
constant voltage output. As the probe is submersed into an aqueous solution, electrical current passes from the reference electrode into the measurement cell, then along a silver/silver chloride wire to the meter. The pH value (in millivolts) displayed is the difference between the measurement cell and the electrical current produced by the reference electrode.”

I can see if electrodes can’t conduct in pure water. Most RO has some inpurities so if it will conduct, it should also read.

On another note I learn something new every week on the forum. That is one of the reasons why I keep up with it after a few years. :smile:


They are measuring a solution. This is also why 1 drop of UP or DOWN can move a large volume of pure water a significant amount.

Seriously; you don’t need to PH pure water. It will adopt whatever PH is present in the medium.

The reason you can’t get an accurate value with low TDS is the water acts as a dialectric and passes voltages more like a capacitor than a proper electron flow. Different methods are used to obtain values when so low. And yes; really pure water is a problem because it wants to reach homeostasis and scavenges elements it is in contact with. Stainless steel etching is a serious problem in commercial R/O systems and usually degrade the purity to avoid this.

It’s kind of an interesting subject. But I’ve been a bit evangelical about it to the point where a lot of growers are not PH’ing their seedling water etc. If I grow in OF I just do straight R/O until I have to add cal mag.


Yup totally agree.
I only measure after additives obviously. Got distracted in the science, but in practice nobody should try to ph before additives since it wont matter anyway. Thanks for working it out with me Myfriends410! :slight_smile:

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Yes it does. I just have a smaller 18 site :+1: