Originally Posted by Soothing Shrimp
Ah! This is why I love this board. I learn sooooo much! Okay, so I just tested my RO and it is around 6.9
The only things I have in my tanks are inert substrate (black blast), moss, heater and air driven sponge filters. No rocks or anything like that.
What is the current TDS in these tanks? What is the KH?
My guess is that the pH-raising culprit is whatever you are adding to your RO water to re-mineralize it.
If you are just mixing RO with tapwater to achieve a balance for your tanks, I would switch to using 100% RO and then adding minerals to raise your TDS to between 150 and 200. This should lower your KH enough so that an active soil could lower your pH. You could achieve this by mixing different amounts of tap water with RO, but it is much easier to get right when using mineral additives.
Good products to add minerals to RO water that will not raise your pH are:
Kent RO Right or Salty Shrimp GH+.
Once you have your KH low enough (0-1) so that an acidic substrate can lower pH, the best substrate to use, in my opinion, is ADA new Amazonia. I have also had good results with ADA Malaya, but prefer Amazonia.