Sand-changing after a Baq conversion is a must. Sorry, but you should have done it at that time. Afterward, you should never need to change it again.
I'm strictly guessing here, with a full-scale WAG on my part, but the sand get full of Bacqa-goo (that's the "technical" term for it) which is why you dump it, and you're using all that chlorine to process THAT stuff that's still in the sand. Amazing that the filter is working!
If you don't mind spending the $20/week for a carboy of LC through the summer, then you'll just have to do that. (assuming my WAG is on target).
Or take a day, empty the sand and replace it and be done with it.
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