I am watter changing with an RO /tap mix and it seems to not want to move down in the beginning.
And thain all of a sudden it will drop, maybe something leaching out of the substrate or whatever
I made a an RO /tap mix in a bucket,,,,
just add say 2 inches of RO and 2 inches of tap 50% mix = 4 inches, now test it
ops need more tap water 6 inches in the bucket =1 part RO to 2 parts tap mix now test it