low alk (6dkh) and calcium 400ppm ???????

[Myka]

For an explanation on how calcium, alkalinity, and magnesium interact with each other take a read through the link in my signature. I also suggest different products, and dosing methods.

Balanced Calcium and Alkalinity:

1.4 dKH to 350 ppm
2.1 dKH to 375 ppm
5.6 dKH to 400 ppm
9.1 dKH to 425 ppm
12.6 dKH to 450 ppm
16.1 dKH to 475 ppm

Personally, I aim for 9 dKH alkalinity and 420 ppm calcium. Generally, natural seawater contains 7 dKH alkalinity and 410 ppm calcium, so you can't go wrong imitating that either.