I just thought that I would share this with you all. If you are using
www.aquariumwatertesting.com for occasional water testing I found this to be interesting when I saw that my salifert was reading 448ppm and there test was reading 212ppm. So I sent an email asking what was up with that. Here is the response.
"Your test kits are probably fine. We use an Ion-specific electrode to
measure calcium. This probe will only "see" calcium ions, or essentially
"free" calcium in the water. It does not see calcium that is in any kind of
association with other elements such as carbonate, bicarbonate, fluoride,
sulfate, ect. Your titration style kit does not differentiate between the
varieties. Corals will use all types, but they prefer free calcium because
it is the easiest to use, but they don't really care. We chose the
ion-specific electrode expressly because it is a method that is not
generally available to hobbyists, in an attempt to give them an additional
piece of information about their water chemistry.
In natural seawater, about 20% of your calcium is found in these
complexed compounds. In aquaria, with the inherent ionic imbalances
associated with aquarium chemistry, as much as 40-50% may be complexed at
any given time. Watching both values for calcium over time, and making sure
that both values remain stable, indicates a more stable environment, and is
more conducive to coral health and growth. You may also note that in the
report that you received, our recommended values for calcium reflect the
amount of expected
ionic calcium, and not the total calcium level of 400
that you are familiar with. Based on those values, you are really not that
far off from natural seawater, and well within the norm for aquariums. I
hope that this helps."