Need help with math

[Mr Fishface]

As the title says, I need some help with my math. I can't figure this out on my own!

My total system volume is about 180 gallons (estimate) and I go through about 2 gallons of top off water each day. I have been testing my calcium in the tank and it has stayed steady at 440ppm for the last 2 months solid. I used my saltwater test kit to test my top off water (unsure how reliable this is though) and read the top off water has 140 ppm calcium. What I am trying to figure out is how much calcium is my tank using a day?

Here is the math I did but I am not sure if I am correct here. If my total tank system is 180 gallons, I removed 1 gallon of tank water and replace it with "nothing" water (ie values are all 0) that is a 0.0055 reduction in values. That means if I replaced a gallon with zero value water I would lose 2.42 ppm calcium (multiplying the 0.0055 with the 440 ppm I have). So losing two gallons of water a day would be a loss of, for simple math, lets say 5 ppm. If my top off is holding values steady then can I assume my calcium absorption rate is about 5 ppm per day?

I think it may be time to upgrade my testing agent. I use the API kit which measures in quantity of 20s so it's hard to tell these small differences with my test kit anyways. Being that this testing has been happening for a long time I would have assumed some change by now. What do you all think?

 

[Dkeller_nc]

Calculating this is relatively easy, but there are other problems underlying the assumptions. First, about the calculation - "ppm" for dissolved substances in aqueous solution is the same as milligrams per liter. If you are adding 2 gallons per day of top off water with a calcium concentration of 140ppm, then you are adding 7.57 liters X 140 mg Ca = 1059.8 mg Ca per day. In a 180 gallon water volume, 1059.8 mg Ca equals 1.56 ppm (1059.8 mg/681.4 Liters).

The issue with the underlying assumption is that a) the calcium test is accurate and precise (they often aren't) and b) your evaporation rate is constant (it typically varies with the outside weather and consequent indoor humidity).

A much, much bigger problem is using top off water that isn't pure water. If you're taking pure RODI and adding calcium chloride to make a 140 ppm solution, that's unconventional but probably OK. If, on the other hand, you're using non-purified water as top off water, you're likely to run into problems down the road because almost all potable water contains metal contaminants that will build up in your reef over time.