I think your spot on with the ALK... A swing from 5.5 to 8.5 is pretty big within a couple of weeks. My tank went from 6-8 and I lost several colonies and almost all growth tips were burnt. I opted to stick with reefers best and reef crystals salt due to the low ALK in the mixture. Fluval is very high in ALK which was my problem.
On a side note, I would do some calculations on how much you should be dosing and measure to ensure it's dosing the right amount. After messing with dosing for a couple of months, I dropped it and picked up a calcium reactor. I would never do an SPS tank without one again.
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