What are you chlorinating with? If it's Cal-Hypo, that's your problem.
Otherwise, I suspect it's in the plaster, and the rocks and morter.
You may want to drain about 3/4 of your water and refill--try and find a water that's not so full of calcium...I don't know if a water softener will help.

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I've been trying to keep track of the CH level of the fill water on a seasonal basis. If I can find a season where the water has less calcium, I'll probably just plan for a yearly partial water change timed to the lowest CH levels of the year.
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