Is calcium and alkalinity depletion a linear function at all levels of calc and alk?

[JHawlz9989]

I'm trying to get a good idea of how much calcium and alk my tank uses. After 10 days, my alkalinity dropped from 9 to 8, and my calcium dropped from 380 to 340. I use IO salt, so water changes barely bring my calcium levels to where they need to be, and as a result, I use calcium chloride to maintain my calcium. I'm going to dose one mL per day and see where I'm at within a week.

My question is, is the usage of alkalinity and calcium a linear function at all levels? That is, since my calcium level was below 380, did the calcification slow down, or even potentially stop, or does it continue at the same rate until all the calcium is depleted from the water column?

My tank is only a 20 gallon, but with new live rock and a small colony of GSP and zoas, I'm wondering if using limewater for top off would be a solution to maintain my levels. It certainly wouldn't be saturated. I guess I'm just wondering if those depletion rates across 10 days is truly accurate.

 

[bertoni]

The consumption of calcium and alkalinity will vary with a lot of factors, including the levels in the tank. As the calcium and alkalinity drop, the consumption rate will drop. The same goes for pH. Of course, those rules only hold over a fairly small range. All in all, the consumption isn't a linear function of time, if that's what you're asking. The two parameters do drop in a linear relationship with each other, at the rate of about 2.8 dKH per 20 ppm of calcium. Magnesium consumption will throw that off a bit.

 

[HighlandReefer]

"I use IO salt, so water changes barely bring my calcium levels to where they need to be, and as a result, I use calcium chloride to maintain my calcium. I'm going to dose one mL per day and see where I'm at within a week."

Many hobbyists find they need to bump up the calcium and magnesium levels for instant ocean mixed to a salinity of 1.0264.

Per Billy's testing and I find about the same parameters myself:

Instant Ocean

calcium: 400
alk: 11 dKH
mag: 1350

So if you keep your calcium level higher, then you will find that you need to add more calcium. Also the same applies for mag.

I find it easier to just add calcium chloride and mag supplement to my salt mix water change to bring these levels up. Then dosing will be much better balanced for maintenance.