I've never experienced it, but regardless, I'd recommend reducing your flow through the reactor for a couple weeks after you change.
If I were to stake a guess, it would be that pre-stage 3/late stage 2, most tanks can't handle the abrupt drops in the levels of organics that generally follow zeolith switchouts. That said, I'm finding it more and more suspect that the FM product is, as claimed, safer to use and less aggressive...
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This and that.
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