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Discussion Starter · #1 ·
Started this tank on New Year's Eve. In the beginning was just using RO but the municipal water supply here is ridiculous, so I added a DI cartridge. Everything ran fine. About a week ago, the product water began running a Ph of 8.0. Still producing 0µs; 0ppm. Plants are doing fine.

Any pros here have any ideas how this could be? Less than 100 gals. run through the RO/DI, so I feel fairly certain that's not the problem. Also spoke to the water department's chemist last week and he had no clue (big surprise) and said they've noticed no significant changes in their daily test battery.

I have the water dept.'s annual report if anyone thinks that may shed some light. I can attach later if needed.
 

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I have been thinking about this subject for a few days, based on another thread here: We are very used to having our water behave in certain ways, including maintaining a reasonable pH even if we do nothing to make it do so. We also constantly hear that we need to keep our KH above some value or we will get a pH crash, because the KH is a buffer - this is incorrect but I think it is a clue. Normal water always has a KH above zero, meaning there is always some amount of carbonates in the water, and atmospheric CO2 keeps the amount of carbonic acid in the water above zero. So, the carbonic acid/carbonate buffer is always there, trying to keep our pH relatively stable.

You ran your water through a RO/DI filter, you might have removed all of the carbonate ions from the water, leaving it without a pH buffer. So, suppose you somehow have an alkaline compound in your water, perhaps from fertilizing or from another substance you added to the water. Without the carbonic acid/carbonate buffering the added alkaline substance caused the pH to go up an abnormal amount. I would have expected to see the pH drop due to the unbuffered carbonic acid from atmospheric CO2, but if the amount of added alkalinity was greater than the acidity from carbonic acid, the pH would go up.

That is just a germ of an idea right now.
 

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Discussion Starter · #3 ·
That's a good line of thinking Hoppy, thank you. I wondered along the same line but still can't see how TDS would still measure zero.

When I reconstitute, using PPS RO Essentials, it measures 180µs/90ppm and a PH of 6.6. The reason I was somewhat concerned is that my tank PH does drift throughout the day. Yesterday for example, prior to CO2 and lights on, PH was 6.8. During peak lighting/photosynthesis hours it read 6.4. Naturally that lead me to wonder about my KH level. Everything seems fine, fish, plants, etc. And I understand this is still a new tank so I'm not panic striken by any means. Doing another water change tonight and will continue to monitor the situation.
 

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Hello, is the RO/DI water’s pH value 8 in the storage tank? After reading your original post that was my understanding. What kind of material is your holding tank made of? Perhaps it’s leaching something into the water and changing the pH. Given that the water has essentially zero dissolved minerals, a very small amount of a substance leaching into the water could result in a noticeable pH change. Also, temperature can effect pH reading so I imagine you’d want to measure the pH of the RO/DI water at the same temperature as your tank water. One final thought, how much do you trust the reading from your TDS meter? Have you calibrated it?
 

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pH 8.0 is fine. If you want it higher, the best solutions are lower CO2 air (fresher air or a CO2 scrubber) or adding a high pH alkalinity additive (best is limewater, second is carbonate alk additives). Another option is using up CO2 with photosynthesis.
 
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