wharyat said:

What are your magnesium levels?

Ditto n the Mg request, there are usually issues with the conservative element proportionality of your seawater when you begin to see issues like this. There is a mathematical term called the supersaturation quotient for your cacium levels with respect to your alkalinity. It is the product of your calcium concentration times your alkalinity concentration mutiplied by some constant factors for the water column, divided by a standard for the water column with respect to a standard for the calcium and akalinity concentrations. If you are at supersaturation, any change to one of these concentrations will increase the total value of the suersaturation quotient and precipitation will ocur if the water is at the max concentration for the conditions in your tank. Increasing either calcium or alkalinity at supersaturation will precipitate both elements in equimolar amounts (equal numbers of molecules) until your supersaturation quotient returns to normal. As an example, suppose your value for the supersaturation quotient is 6.0 at supersaturation, and the value for the Ca = 0.425 and the dKH = 14.1 (not real values, but I included coefficients, etc. in these for an example only, the math is much more complex than this) :

- 14.1 x 0.425 = 6 as the supersaturation quotient (remember, not the actual numbers)

If we increase the calcium, it will raise the conditions within the tank by that amount, and until the calcium comes back down as calcium carbonate precipitates to values for the dissolved ion product for a value of 6, then it will continue to precipitate:

- 14.1 x 0.500 = 7.05, precipitation will occur until it reduces the supersaturation quotient to 6.0
- 13.25 x .450 =~6.0 (actually 5.9625) which ends up dropping our overall alkalinity, but increases out Ca ion concentration. A fine powdering of CaCO3 snow will appear in the tank

This will continue unless the ions are added in balanced amounts to prevent this phenomenon from occurring (see the Kiedemani cacuators for this determination). Other factors may affect your water column's ability to maintain supersaturation, such as drops in salt concentration, changes in Mg concentration, or other related issues. They will actually drop the ability of the water to hold supersaturation, making the quotient drop even more.

We need to test your water column for pH at the beginning and the end of your photoperiod after performing the borate (20 mule team borax) test for the probe to see how well it is actually measuring pH.

Get us a value for your Magnesium ion concentration using a salifert test kit, an accurate answer for the borax test, then the values for your pH at 7am and 7 PM (or just prior to lights coming on and just after they go out), as well as a refractometer value for your salinity. This will determine how much your pCO2 value is (intimately linked to these values), and will determine if this is a factor in reaching your supersaturation. It will help to know bout how often and how much additive you are putting in the tank on a reguar basis (no guessing, if you don't know, state that as an estimate)

Looking forward to your responses, see the following for more info:

http://thereeftank.com/forums/showthread.php?t=67560
http://thereeftank.com/forums/showthread.php?t=66265
http://thereeftank.com/forums/showthread.php?t=65873
HTH