Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Test the RO water. Is it pretty close to 0 for everything you have a test for? An old or faulty membrane might allow something through, so replacing the membrane would be a step in the right direction.
If the RO unit is OK, then find out where the GH and KH are coming from.
If they are separate sources, then remove that source.
If they are coming from the same material (Coral sand, limestone rocks, any of several other things) then you will have to remove that, then correct each GH and KH back to what you want.
To find out where these minerals are coming from put a sample of everything from the tank into separate jars or glasses with RO water. Include a test jar for each of any fertilizers that you are using, especially Epsom salt, trace minerals, or anything that claims it adds calcium or magnesium.
Test at 24 hours and 48 hours. GH, KH, TDS, pH.
Once you remove the culprit, see where the water stabilizes (there might be more than one thing contributing to the problem). Hopefully, with the major problem solved, the water will be a lot softer. If you need to raise either or both GH and KH:
GH booster to raise the GH.
Sodium bicarbonate (baking soda) or potassium bicarbonate to raise KH.