95GalReef -- I think DEADFISH may be on to the source of your problem. In the absense of any other leads, I suspect your tank may be badly deficient in free Magnesium ions.
You ask where your Calcium and Alk are going? Well, in the absense of Magnesium, they are going to the bottom of your tank as solid Calcium Carbonate. And without Magnesium, no amount of Calcium and Alk building products will change that.
If you don't already have one, get a
Salifert Magnesium test kit. Follow the directions, and you should be able to drip almost the entire syringe full of the final reagent into your water sample before the color changes to blue. If it changes sooner than that, you need to add Magnesium. Use something like Seachem reef Advantage Magnesium or Kent Tech-M to boost Magnesium to 1500 ppm. There are probalby lots of other suitable Magnesium raising reef products out there, too.
Once your Calcium and Magnesium are in balance with each other and with your Alkalinity, pH will stabilize around 8.0-8.3 with no other action on your part. The pH problem you see is just a symptom of not enough reserve buffer capacity for pH stability against the organic acid production load in your tank, imposed by your livestock.
Some Background: Natural seawater has more than 400 ppm of Calcium, as much 425 ppm or even 450. But this is only possible because seawater also has over 1500 ppm of Magnesium.
Reef keepers often forget that Magnesium is the key to Calcium solubility in water with high alkalinity -- 2.86 meq/L alkalinity (dKH 8.0) and above, like in our tanks.
One definition of alkalinity is the buffering capacity of the system against acid. Most of the buffering capacity in natural seawater is from HCO3- and CO3--. The concentration of CO3-- in the water directly influences the saturation state of the water with respect to Calcium ions -- and Magnesium ions, too.
I try to run at 3.6 meq/L (10 dKH), so to keep my calcium above 400 ppm I have to keep my Magnesium above 1500 ppm.
You might ask, how does Magnesium make Calcium Carbonate more soluble in water? Well, Calcium and Magnesium are the most prevalent metal ions in seawater, and both form carbonates by bonding with free CO3--. This is called the "common ion effect:"
Ca++ + CO3-- <==> CaCO3
competes with
Mg++ + CO3-- <==> MgCO3
Since the Mg++ and the Ca++ both compete for the attentions of CO3--, Magnesium ions can help keep Calcium ions in solution.
The "solubility product" (denoted Ksp by chemists) of Magnesium Carbonate (normalized to 25 degrees C)is 6.82 x 10-6, higher than that of Calcium Carbonate, CaCO3 at 4.96 x 10-9. So, we need lots of Magnesium around if it is to out-compete Calcium for free carbonates.
Without free Magnesium ions, your Calcium will tend to precipitate out as solid Calcium Carbonate, CaCO3. The more you try to raise your Alk, the more Calcium you will use making useless Calcium Carbonate (ie, sand). Then in frustration you dump in more Calcium, then more Alk builder, then more calcium -- sound like what's happening in your tank?
[ July 26, 2001: Message edited by: BReefCase ]