pH rising in RO water tank

[MBStevens]

I dedicated a tank to German Blue Rams (not received yet). My natural well water is super high pH and kH (8.6 pH and off the charts kH), so I decided to get a small RO/DI system which has worked well. I filled the tank with majority RO water, and only a few millimeters above the substrate of my well water (over 90% RO/DI water). For awhile, the tank seemed to be holding around 6.8 or 7.0 pH, with about 40ppm kH, but now it seems to be going up to 7.6 or 7.8 pH with 80ppm kH. WHAT THE HECK. I just want to keep rams and I want to provide the best possible environment for them. I can't figure out why my pH is creeping up like this. I understand that those parameters in themselves are not unreasonable/unacceptable for rams, but I do not want pH swings when I do water changes with RO water. I don't want to have them going from 7.8 pH down to 6.8, obviously. That is my main problem, although rest assured I sure do want my 6.8 or 7.0 pH since I spend money and effort getting RO water pumped out!

 

In the tank I have driftwood that leeches tannins, a small sponge filter with airstone, plants, dose Easy Green and root tabs, and have plain black gravel for substrate. I also have some rocks in there. I got both types (I don't know the name) from local fish stores. My guess is that those rocks could be raising the ph/kh? Is that common with normal rocks used for aquascaping? I want to have a pretty tank...but now I'm wondering if I should take them out and see if it goes down? 

Edited December 27, 2021 by MBStevens

[Gator]

@MBStevens; I think it would be a great idea to remove the rocks, check your pH, check your pH again a couple of days later, if your pH is still high after a couple of days, do a 25% water change using the RO water. After you get your pH down, place one of the rocks back into your tank to see if your pH goes back up, if it does, the rock is probably Limestone, which will raise pH, remove it if your pH goes up.

[Mmiller2001]

It's 100% the rocks or gravel raising the KH. Ignore the pH, and find whatever is raising KH and remove it. Replace it with inert stone if you want to keep that look. It's a wives tale that pH fluctuations harm fish. 

If soft water fish are your interest, like mine, I would just run 0dKH. Forgo the mixing and just use your RO water and mineralize the RO water to 5dGH and call it a day. MgSO4 and CaSO4 are readily available and dirt cheap and I would mix 2:1 Ca:Mg.