I probably won't have time to go to the LFS until this weekend. But now I am suspecting it maybe combination of bad testkit and substrate.
My substrate is flora-base like material, and on top of it are those colored gravels. I know, I know that it's unnatural to find color gravel in the wild. But I thought the grain size is suitable, so I just used it.
I wonder if it's the gravel that is the culprit? I measured the KH again this morning (about 4 hours ago), the KH has risen to 9 degrees. I backed off CO2 a bit last night and pH went up to 7.0. So I am still at 30ppm-ish of CO2.
Maybe I should look into replacing all gravel to plain sand, with flora base material mixed in. Other than gravel, I don't think there are anything in the tank that could've contributed to KH rise. I do not use any filter material other than polyester filter floss, and the decor is bog wood. Could flake food contribute to KH? Kind of doubt it.
Oh, decision, decision, decisions.
Do y'all think I should continue to do more frequent water changes to bring KH back down, until I get the gravel replaced? Because by the look of it, it's going up 1KH a day, and that is a bit scary.