Calcium depletion

[Drs1685]

So let me start out with my coral stock list, 1 head frag torch coral, I head frag frog spawn, 3 heads hammer, 1 Lt plate, I clam, 1 lg colt coral 1 lg sinulara, 1 lg gonipora, 10 purple mushrooms, 15 heads misc zoas, 2 in brain coral, frag candy cane, chalice frag, and a few other small frags in my 75gal, I started testing my calcium and alk levels 6 days ago everyday at the same time after top off to see how much I needed to supplement. My calcium has been 500mg/l (25 drops on API test) and my alk has been 9dkh(9 drops) API test, these results have stayed the same all week just today the calcium has seem to drop to 480mg/l (24 drops) alk has stayed at 9, ph is 8.4 btw. My question is does this seem normal I would expect to see my calcium drop a bit more in a weeks time alk as well.

 

[ccCapt]

The API calcium test kit only measures in increments of 20ppm (not very accurate). With that being said, there is no way to know that your calcium was actually 500. It could easily have been 490 or 510. The same goes with your new reading. It could easily be 490. API test kits are cheap, and for good reason. They are not very accurate. If you are really concerned about your calcium and alk levels look into getting Salifert test kits, but they cost considerably more than API and are also considerably more accurate.

Just a little FYI....
When calcification occurs (corals forming their hard skeleton, coralline algae growing or abiotic precipitation) they is a defined ratio of calcium, alkalinity and magnesium used in the process. That ratio is aprox 20ppm calcium - 2.7 dKH alk - 2 ppm magnesium. So if your test shows a 20ppm drop in calcium, you also dropped 2.7 dKH in alk and 2ppm in mag.

 

[Drs1685]

So let's say my levels were 500 and dropped to 480 in a week does that seen normal or low?

 

[needmorecowbell]

It all depends on the amount of corals in your tank. As ccapt said, if it dropped by 20, look for a change in alk, it should be lower by a specific number. If it isn't, try getting your mag level steadier. It would mean that the carbon and calcium are bonding in the water column.

 

[ccCapt]

So let's say my levels were 500 and dropped to 480 in a week does that seen normal or low?

There really is no normal, high or low. It depends on the tank itself and everyone's tank is a little different. Did you also see a drop in alkalinity? If it was 9 dKH it should have dropped to around 6.3 dKH. If not, it's more than likely just the test kits inaccuracy. Just by looking at your stock of corals I would think it's the test kit. If you have coralline algae growing rapidly, it can also deplete the calcium/alk.