Hi Flora,

I understand your point, but test kits don't alsways have to be calibrated..........there are two types of calibration.........ablsolute, which in my case is useless (although I can do it because I do have reference solutions) and relative which in my case is all that is needed..........My test kits test test 0 for RO water and 4 for 4dKH reference water......if you want I could do as Tom said and dilute the 4dKH to 2dKH.....I also have 5dkh that can be made 2.5dKH...........My point is that in my case I would never ever have to do this.

I am looking at a relative value........I take a sample of water right before the w/c and a sample right after the water change and then take the KH of the two samples at the same time. the first sample turns blue in exactly double the amount the second sample turns blue........relatively speaking, the first sample has double the amount of KH (regardless of what this KH is) as the second sample.

My simple question is what is causing this KH to double?

Tom's guess is something leeching from the substrate, I can buy that.

People on this forum are so uptight about testing and test kits.......everyone needs to relax.

Ken Takeuchi