Could be. I quit putting acid in, added 4 lbs. of baking soda and then did
4-6 water changes of 20-25% over 10 days (this was recommended by the
experts on the KHA board) it worked. Plants were happy after that. Frogs
never were bothered, amazingly. ~ jan

Oh, the acid went in the water. Still, isn't acid etching what one does
to open the pores of concrete for adhesion of various coatings? But, you
obviously know what you're doing and how to correct the problem if it
continues.

I can only hope (I know what I'm doing) some days it's definitely
questionable. ;o) ~ jan

~Power to the Porg, Flow On!~