IO Alk Problems

[midgetwaiter]

I've recently switched the fuge on my FO tank to xenia rather than macro and it hasn't been doing so well. I don't regularly do testing of the system other than nitrate because it never really mattered before now. Last night I checked out my water params and found my pH at 8.6 (electronic and Elos) and my Alk at something stupid like 20 dkh (Sailfert and hagen). Calcium is a little over 500 and Mg is 1300.

I use tap water in this tank so I figured I was seeing the result of too many top ups and not enough water changes (done that before) so I mixed up 40g of water to do several partials over the next week. I tested the new water and found the alk was really high again, right around 20 dkh.

I figure I have a really screwed up bucket of salt here but part of this kind of bugs me. My understanding of the alk / calcium balance leads me to believe this shouldn't even be possible, the calcium carbonate should precipitate out at these levels shouldn't it? Or is the Mg holding it in check?

Anyone else seen this before?