Alk doesn't move one week, dives the next

[Uprising Rage]

Why does my tank alk stay the same one week and then will go down 2 or 3 dkh the next week. Makes it impossible for me to use kalk. Does anyone know why my tank alk, isn't steadily going down like calcium does?
It's the same livestock. I Have a 20 g long, with a 20 g sump, skimmer and cheato. I have tried to find an answer online but no luck.

 

[marrone]

What do you have in your tank? Alk and Cal play together, so what is your Cal reading during this time?

 

[aaron23]

are you using any ferric oxide phosban media?

 

[theMeat]

Prolonged kalk use can cause some alk issues. First thing, especially since you grow macro and macro consumes magnesium, is chech your magnesium level.
Does ph go up and down with alk?

 

[Wondering]

Have you seen this article by randy holmes? He addresses this issue in the article

http://reefkeeping.com/issues/2004-12/rhf/index.php