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The biggest difference in lithium and dichlor is that dichlor contains CYA and lithium does not. They are both chlorine. Lithium has a higher pH and you would need to use more product to get the same free chlorine readings which would add to TDS faster.
Is the foam simply soap residue from suits .....
With the Brilliance Bromine that I use (contains no chlorine) and their MPS shock, PH does tend to drift low over time, but it's easy to get back in range using Ph increaser stuff. Not sure if it's the shock or the bromine doing that. I don't see why using dichlor would be a problem with kids- chlorine is used to sanitize swimming pools, and that doesn't seem to be a big issue anywhere.Brewman