The levels are fine I was just wondering because it couldn't hurt to use a tiny tiny bit of kalk in the top off but what I'm doing now is working. I have to keep an eye on it now since I got the second clam though.
If the levels are fine, I'm always one to not "rock the boat". Watch your levels. If the second clam results in a dip, see if a larger water change fixes it first. IMO, some tanks are too small to mess about with dosing.