Do I have a faulty Calcium Kit?

Hey All,
I'm slowly expanding my testing arsenal and am wondering if i've got a faulty Ca test kit. I have never dosed and calcium in my tank before, only what comes in with water changes in the salt. Have been using Instant Ocean salt unless something is on sale. I have tested my water several times and have been coming up consistently with around 480-500 ppm calcium. I have a few SMALL sps frags (Monti caps and Acro) and some LPS ( about 12 heads between duncan, Frogspawn, hammer, and candy cane) Do you think this is a normal number or is there something wrong with the test? I have had quite a few chemistry classes over the course of my high school and college career so i'm fairly confident i'm testing correctly. (Everything is room temp, every thing is mixed with the first reagent and i'm allowing time between drops for the second chemical to disperse. I keep coming up with a number about 500. I know i shouldn't have a huge demand for carbon in the tank but i would think that by just doing 5 gallon changes every other week on my system that is about 75 gallons i would see a lower number? Maybe i should try testing the water next time i do a change and see if there is a difference between the new water and my tank...

 

When you do those water changes, you're adding calcium. You'll still end up lower, eventually, but it sounds like you have low calcium demand judging by your corals. What size tank is this? Also, what kind of kit are you using (in particular, what kind of resolution does it offer? 50ppm? 10 ppm?)?

 

Its a 46 bowfront with 29 gallon fuge. Its and API kit and its 20 ppm between drops of the second reagent. IK the water changes will introduce calcium, i just didn't think Instant Ocean would give me that much calcium...

 

Well, if you have a demand of .5 ppm per day on a total real volume of 50g (Your tank is not 75g water volume), a 5g water change every week with calcium level of 500 will replace enough to maintain you at 480+ for a month (almost definitely longer). With such a low coral load, assuming no increase in usage (corals don't grow, and you don't add more corals), you could stay at 470 pretty comfortably over the long haul (28 weeks).

I even made an excel calculator for this that I wish I could share; feel free to use and share it once I get it up. It works for alk too; just gotta replace the numbers with ones appropriate to alk. I can't attach it to this post, though, dunno why.

 

Makes sense. Thanks for the help!