Can't maintain alk...

[MikeP]

I've never been able to keep Alk very high in my tank. Over the weekend I did a water change and brought Cal, Mag, and Alk levels up to where they should be. Calcium is at 440, Magnesium is at 1350 and Alk was at 9.8 (I'd like to bump it up a bit more but that seems to be as high as it will go) I tested for Alk yesterday morning (9.8) and then again that night (about 15 hours later) and it was down to 8.4 but other levels were still the same. Why does my Alk drop so fast?