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Stabilizing lower pH


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I've generally tried to do as much as I can in tap water to avoid the extra maintenance and difficulties that can come with trying to 'chase pH', and have had relative success despite liquid rock coming out of my tap (pH 8.7 after 24 hrs, GH and KH 20). I'd really like to venture out into some species that require softer, more acidic water though, especially for breeding. I've been experimenting in empty tanks and seem to be able to achieve my target values when first dosing, but can't keep those values stable. My approach so far has been:

  • Create 0 TDS water with an RODI system.
  • Add Alkaline buffer to reach target KH. 
  • Add Acid buffer to reach target pH. 
  • Add Equilibrium to reach target GH. 

For my initial test, I targeted pH of 7.0, GH and KH of 5-6. On day 0 (i.e. shortly after mixing), my tests showed that I'd hit each target within the accuracy of the test. I continued testing pH on subsequent days though and saw a steady rise for about a week before finally stabilizing at ~8.2. This is a new tank with literally nothing in it except RODI water with the above additives. Is gas exchange somehow causing that significant of a shift? Is there some latent interaction with the buffers, or some other cause I'm overlooking? In recommendations for how to edit my process so the resultant water has a more stable pH? I'm not as concerned with the exact number as long as it's stable and somewhere in the slightly acidic to neutral range (maybe 6.2-7.2).

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I'd say co2 in the water is the culprit. It's quite common in tap water especially with high pH where the city is trying to keep it lower for "safety" of pipes. 

Also as for lower ph, I'd never target anything lower than 7.0. The only species I've ever bred that required it was crystal shrimp. Every other apisto and other fish that "needs lower ph and softer water" has spawned at 7 or higher for me that I've tried. 

I'd be using a mix of R/O water and your tap water to get desired results. Or just setup tanks that don't get water changes, they'd acidify over time, plants would remove minerals. Then you'd only top off with tap water to replace the minerals and maintain pH. Could do the same with start with R/O water, but would need to be much more careful at the beginning making sure the plants can keep up.

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Thanks all!! I think I got too caught up in the 'RO removes everything' mindset to differentiate between gases and solids. Seems so blindingly obvious now. I'll add an air stone to my RO storage and redo some tests using aged RO water, then report back in a week or so with results. 


@Cory I like your suggestion of a no water change tank in order to keep the pH down. I've been trying to move that direction anyway. I have a 100% tap water planted tank that's only been getting a water change every 2-3 months, and the pH in it has stabilized at about 8.4 vs the 8.7 of the tap. I'll try establishing something similar but starting with a tap/RO mix to see if I can get a stable lower pH, ideally with no water changes except for top off. Are there any particular tests you do to make sure mineral levels don't get too low for the plants? Would something like a GH test be sufficient or would I need to test for more specific things like Calcium?

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