Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Could there be nitrates in the tap water?
Do another water change, but vacuum the substrate as much as possible. If nothing else drain the water through the substrate by digging a hole in a corner of the tank.
I also have had nitrates seem not to go away very well with a water change, even when the tap water had zero nitrates. Then I would do a water change with the goal to rinse the substrate somehow, anyhow, and the nitrates went down, as if there was some way they sort of hung up in the substrate during the water change, then get released into the new water when the tank is refilled.