Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
RO should have nothing that alters the pH, so the 6.8 you are getting after aerating it is typical. RO water will take in CO2 from the air, and the pH will stabilize somewhere under 7.0. Then, adding carbonates (baking soda) will indeed raise the KH and pH.
I am concerned that you are testing 2 degrees each GH and KH in the RO water. Which test kit are you using?
Also, the TDS meter is showing zero, but there is something in there giving you GH and KH readings? Does not sound right. I wonder if the test kits for GH and KH are off.
The pH change with that small dose of baking soda is extreme.
When I dose that much baking soda I am only getting a shift from about 6 to 6.2 (3 different testing kits), and the KH comes up from zero (2 different testing methods) to 2 degrees. This is not quite the same as you got. 1/4 tsp in 7.5 gallons ought to give you 2 degrees of KH. You got twice as much change in KH. Either your 1/4 tsp was mounded up, or the test kit is not accurate.
Oh well, lets say that is just what happens, so far.
Now, do you have any source of peat moss, oak leaves, alder cones or almost any other clean, pure organic matter? Not potting soil that may have other ingredients, and read the label on the peat moss to make sure it has no fertilizers or surfactants.
Add a handful to the 4 gallons and stir. Let it sit overnight and test. The pH should drop. The GH and KH should not, or maybe the GH might come down a little. A small enough change that it is hard to see, in most cases, though I do remember one post from someone who saw a big drop in both GH and KH from adding peat moss.