Adam's GC-MS ICP is a much better machine .  Why buy cheap?

My kit is "more accurate" only because I'm doing the titration using a burret.  I can resolve the volume of EDTA titrated down to 0.01 ml... which is equilivant to 0.8 ppm Ca in the water.  That accuracy is not really needed though, imo.  There's also the inaccuracy of determining just when the endpoint (color change) is met.

Yes, I test my water at work.  But HACH does sell a kit:  http://www.hach.com/hc?L3NlYXJjaC5wcm9kdWN0L2ludm9rZXIkRGV0Y WlsL1ZlbmRvckNvZGU9SDIvUGFja2FnaW5nQ29kZT0xNDU3MDEvVmlldz1QU k9EVUNUX09WRVJWSUVXL1BSRVZJT1VTX0JSRUFEQ1JVTUJfSUQ9SENfU0VBU kNIX0tFWVdPUkQvU0VTU0lPTklEfEJrRXpUWHBCZDAxVVNUQk9SR016VGtOY WJtUlhWbnBrUld4TVZIYzlQVUV4Y0V4TlZBPT18

If I didn't have what I have at work, I'd use the Salifert kit.  It's resolution is at 10 ppm Ca.  Plenty good.  My father in law just bought a kit (Red Sea, I think) with resolution at 50 ppm per drop!  That's terrible imo.

I think every Aquarium Systems kit (not just calcium) use repackaged HACH chemicals.  Anyway, I've gone on too long... Marcus, I wouldn't buy the HACH, the salifert is cheaper.

This is a very interesting topic.

Regarding calcium test kits.  I use a HACH test "kit".  Kit is in "" because it's more of a procedure then a kit.  Instead of counting drops, I titrate directly from a burret.  More accurate (and it makes me feel like a chemist). 

This is how the HACH kit works and how, I would hazard to guess, all calcium test kits work (at least the ones I've seen/used):

1. The sample is made alkaline (pH 12 to 13) by adding potassium hydroxide or sodium hydroxide (KOH or NaOH) to precipitate magnesium as magnesium hydroxide, Mg(OH)2.
2. An indicator is added that combines with any calcium to form a red color (or some other "calcium present color", red is most common).  This is ussually some colored looking salt or a drop of a colored solution.  In some hobby test kits the color indicator is combined with the chelate used in the next step.
3. A strong chelate is then titrated (dripped) into the test solution.  The most common is EDTA, but I suppose others might be used.  The chelate is a very very good scavenger of calcium.  It first grabs up all the calcium not associated with the color indicator used in step 2.  Then it starts pulling the calcium from the color indicator.  When all the calcium is extracted from the dye, the color changes from red to blue.  That is the endpoint, all the calcium in the solution has been sequestered by the EDTA.

That's it.  It's just a math problem from there. 

I suppose if the calcium is very strongly held by something, i.e. held covalently... chemically connected not just complexed with another ion/molecule, then the kit would not see it.  But I wouldn't think it would be available for coral growth either.  But I don't know... that's what research is for.  Not sure regarding the comparison of our tank water with actual reef water.  Most here add extra calcium and carbonates to enhance coral growth.  So it's not apples to apples... or did I missunderstand you're idea?

Jon and Marcus,

That is exactly my concern.  Now I also use EDTA every day, but I didn't know that it was common in our home based test kits.  Thanks.  Now I have to rethink. 
Here was my concern before.  What if our test kits are testing low?  What if we have lots of calcium but they are alreadyin incorporated in organics or in viruses and the like?  Your point makes me think the other way, what if our test kits are testing high?  What if the EDTA is falsly elevating the levels of AVAILABLE calcium.  Hmmmm. 

I do have a solution, what if we used our home test kits to test our water, and to also test water straight from a reef?  We couln't quantify anything (since the accuracy of the testing instrument is what we are checking) but we could qualify the calcium levels and see how differnt our water is from the reef water.  That would be sketchy but possibly useful.  I don't think we could prove anything is, but we could prove things that are not.

Adam

Samoa this summer (I hope) and yes I will need several people.  Samples will be collected but not tested on site.  They will be tested by mass spec once I return.  No particular reason for Samoa other than Spargur got me thinking about it and my local Samoan tour guide has impressed me recently.  Testing trace metals, Cu Fe Pb etc.

Oh, Adam, if you do indeed travel to Samoa and need an "assistant" to catch water samples, carry your scuba gear, whatever... I'm your man !

On a more serious note... what metals are you looking for, how are you measuring them, and why is this a concern (is it specific to Samoa?)?