Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
When most of everything is removed from the water via RO there is very little remaining to dictate the pH.
A little trace of anything can make quite a difference in the pH. A TDS under 10 (but not closer to zero) suggests something is still getting through, or perhaps there is something in the collection barrel.
If you leave this water out, especially if you circulate it, does the pH drop? The RO water can pick up CO2 from the air and this will lower the pH.
If you add something that will lower the pH (perhaps some peat moss) does that lower the pH?
What other tests do you have? (GH, KH, other?)
Do you know what the water company adds to keep the pH up? (Call them and ask) Then look that up to see it it is removed via your RO unit. Most things are, but yes, there are a few things that can make it through the membrane. For this reason other filters are often used either before or after the RO unit.
Check with the manufacturer, perhaps there is something you can add to your system. If it is something simple, perhaps you could add a HOB filter on your collection barrel and just run activated carbon or something in that overnight.