Calcium, dKH and mag help

[Kaso78]

I've been doing a lot of reading and for some reason I still can't find a simple solution to my question. I started with a fowlr tank but when the lfs closed up they had a massive sale so I bought a few items 2 mushrooms, zoas and an nem. I had been measuring my cal/dKH levels and noticed a decline. So far the levels are within normal parameters with the calcium at the low end now 400. I have been doing water changes but that's just slowing it down . is there a product that can add without screwing up the ratio? Like a dummy solution to this?

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