Need help urgently: Calcium and Alk levels dropping in SPS tank!

[ml708]

Hi Everyone,

I was a member under a different nickname some years back and now have returned to the hobby!

There is a problem with my 100g SPS tank. Calcium levels can drop from 420-350mg/ml, and alkalinity level drops from 4.5-3.5meq/l in one day. This is worrying as I do not know the root cause and if I keep topping up the calcium and alkalinity levels so rapidly, it will kill my corals and my wallet.

Some background info on the tank

3 acros around 4 inches high and 1 LPS duncan coral (quite big around 30 heads)

Some hermit crabs, sea squirts and feather duster worms

pH 8.3-8.4

Salinity 1.0265

temp 26.2-26.7

Nitrates 0

Phosphates <0.08 (my test kit shows very little deviation below this level)

magnesium 1440 (alittle high I know)

additives (ultra min fine food for sponges, phytogreen 10-15micrometer phytoplankton 1ml/day, polyplab polyp booster 1ml/day, ultra minS nutrition concentrate 1ml/day)

I usually add the calcium and carbonate powder to the sump. Measurements in the main tank and sump have no deviation.

Could anyone please advise?

[Firestarter]

A cheaper alternative is to dose kalkwasser. But do look out for increased pH levels.