Well, your LFS is right (for once
). Seachem products do tend to bring your alkalinity down but that is something that can be corrected. I dose Reef Advantage to my tank once a week and what I do is I buffer all the makeup water. This way, buffered water is slowly introduced to the tank keeping the alkalinity at a normal level. Also, by introducing it slowly, chances of having calcium precipitation are minimized unless you alkalinity goes way high.
I use Kent Marine Super Buffer for the make up water and have not had any problems at all.
HTH,
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Carlos
Carlos' Great Barrier
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Please, take care of the ocean and its inhabitants. They're all we've got!