RO water going acidic

Hi to all,

I hope everyone is travelling well. (or not travelling and well)
I set up a RO water storage container for my room a while back. It is food grade plastic 200 litres. It is not sealed at this point and I have a pump that switches on every 6 hours for 15 minutes. The pump pumps the water out, through a UV filter then circulates back into the tank. The pressure also feeds a float valve in my res for my RDWC setup. As it was RO only i wanted a way to keep it sterilised while stored. I then manually fill that storage container about once a fortnight.
My issue is that i was not aware that CO2 can pass through a RO filter and then over time react with the free H+ ions and create an acid that lowers the PH. From what I have read it usually drops it to between 5 and 6. Not a huge amount. I am finding that my RO store PH is going down well into the low 4s. It takes a lot of PH up to correct it. It seems to take time though to drop all the way down. Maybe a week to go from about 6 down to 4.
Are there any other actions that could be causing this?
I also read that due to the lack of substance in the water that the H+ ions react with the carbon dioxide. I therefor tried to calmag my water while it was stored in the tank thinking that it would help but it does not seem to make any difference.
I would really like to understand what is happening here and hopefully figure out a way to stop it occurring.
I am fearing that this may be something that will prevent me being able to store water before use which is required for my system.
If anyone has any ideas then please feel free to chip in.


RO filtration removes the alkaline mineral atoms from the salt molecules causing the water to become acidic.
Reverse Osmosis takes everything out of the water, that’s why water companies put minerals back in for flavor.

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CO2 forms carbonic acid which can significantly drop pH in water with near-zero alkalinity.

Alkalinity (KH) is resistance to pH change and is distinct from solution pH.

You need some mineral ions for this, mineral ions which increase pH usually as a result.

Try 50/50 tapwater and R/O. It should give you some alkalinity to prevent pH crashes. You can remineralize R/O easily but then what’s the point of R/O to begin with.

Easiest way to quickly increase is baking soda but it’s got way too much sodium for my tastes. Other carbonates like Potassium carbonate are better, and calcium carbonate dissolves poorly in water.


What if his tap water is well water that has a low pH, or is well water that’s treated with salt or chemicals?

@BudSy, were you having issues with the RO water sitting in the tank before you set up the pump and UV filter system? I’ve used RO water since around 2007, always keeping about 25 gallons in a storage container with a float valve, and I’ve never had issues with it just sitting there. I take water from it often, so it never sits for more than a day or 2 without being replenished, which also agitates the water.


Rare for well water to be acidic here in the US. Rarer for it to be treated, and all that entails. Unless you have a water softener but that’s a whole separate thing.

No matter how bad it is, if you can drink it, you can mix it 50/50 and grow fine plants.

My tapwater sucks and I grow fine plants. pH is rock stable, though.

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Rare? Wow, now I feel special :sunglasses:. Mine is below 4.7, so low that it requires a chemical injection system pumping a potassium bicarbonate solution into the incoming well water. If it were to be left untreated, that acidity would leach so much copper from my water lines, that it would create pin holes in them over time.

I think it’s more common than you think. People who live in rural mountainous area or in a rural desert climate, where limestone is non-existent and city water isn’t an option, will often have acidic water.

I miss having city water. It made growing sooooo much easier and more successful.

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Hi to all so far:

@Purple-N-Hairy you raise a good point. I did not provide enough information. My tap water is not too bad. It has an EC of 0.6 and a PH of about 6.5 to 7.0. Once it is run through the RO filter it has a PH of about 6.5 to 7.0 from memory. So the acidity is occurring once it goes into the tank. I do not know if leaving it without the circulation makes any difference. I set up the storage tank, pump and UV filter all at the same time. I will turn off the UV filter and change the timer on the pump so it only comes on long enough to keep the hydro res topped up and see if it makes any difference. The fact you turn over your water so fast means you wont have the issue if it is a common thing. Mine takes a week at least to drop.

If that doesnt work I will look into @vernal s option to do a half and half mix although I was really hoping to have pure RO water to give me more control over the solution. I am probably using the wrong keywords but I am finding it hard to find good information about this on the internet.

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Even weak carbonic acid can wreck pH of your water if you have near zero alkalinity. I doubt your UV filter, pump, or tank is making water acidic.

Why use R/O if your tap is fine? I’d kill for 6.5-7 out the tap.

@Purple-N-Hairy aproximately where do you live a sulfur spring haha? 3 states I’ve lived in and I’ve never had water under 7.7 out the tap.

Aquariums taught me water chemistry some critters and plants are very sensitive.

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No sulfur springs near me, @vernal. Though I do have a friend who lives on a lake in Ohio who has well water, and it’s the absolute worst smelling and tasting. Clearly a sulfur issue - it smells worse than rotting eggs.

Did you have well water in these 3 states?

I agree with @vernal. Have you grown with your tap water and getting bad results?

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Add a few mL of pH down to your tapwater, and the same amount to the R/O water. Test each one this will illustrate my point.

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Illinois, Michigan, and Colorado. Colorado was by far the best. Slightly hard.

Michigan water is famously bad haha. Least we ain’t Flint.

Iron-y water tastes like garbage too.

My grandma had amazing well water. Tasted almost sweet.


For sure. I had some weird issues going back and no way of knowing what the 0.6 EC in my water consisted of. So to rule out toxins i added the filter. Seeing as i have it I am using it. I might just start using the RO filter for watering the organic pots only and try tap water only for the hydro. I am running a sterile res so no concern about chloramine or however you spell it.

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The EC was likely carbonates.

Overly hard tap water has issues, but so does incredibly soft water.

IME slightly hard is best.

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Ok, your grandma had good tasting well water. So did mine (great grandmother, actually). It was in Kentucky, and it was so long ago and so rural, that they had an outhouse and a hand pump for the well water at the sink. But it sounds like you had municipal water in all the areas you lived. Municipal water will almost always be treated to be pH neutral or alkaline.

Speaking of Colorado, I just read last week that Denver (I think) is increasing their target pH for their municipal water to between 8.8 and 9.2! :weary:

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Often they force out the CO2 gas to artificially increase the pH within the pipes.

Looks like most places I’ve lived sit over limestone aquifers.

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:astonished: for municipal water!

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The well water around here has a slightly sweet taste to it straight out of the ground.
The city water here is not sweet and has a pH level of 8.1.
Most municipal water systems run their water pH high because it’s easier on the pipes in their system, giving better longevity than using a lower pH.

Many pipes are lead, too, which leaches lead in acidic conditions.

Exactly what happened in Flint.

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