Cause of rising salinity

[saltyhog]

For the last 6 months or so I've battled rising salinity levels in my 72x24x21 tank. Current SG is 1.027 despite removing tank water regularly and allowing the ATO to replace it.

I top off with RO/DI water via my ATO and 16 gallon reservoir. I dose calcium chloride and soda ash for calcium and alkalinity supplementation via dosing pumps. Due to a rapid growth rate in my SPS/LPS I'm up to 265 cc of BRS soda ash/day and 277 cc of BRS calcium chloride/day.

Is the rising salinity the biproduct of the calcium chloride and the Na2CO3 from the dosing?

 

[Js.Aqua.Project]

So I am not the chemistry master but I'm thinking the chloride in your calcium chloride is bonding with your sodium in the Soda Ash Sodium+Chloride= Sodium Chloride aka Salt.

I'm not sure if it's as straightforward as that but my logic brain says it should be as the Calcium and Bicarbonate will pull off to become Calcium Bicarbonate aka the main molecule in a coral skeleton.

Hopefully the master @Randy Holmes-Farley can confirm if my high school level of chemistry makes sense or not.

 

[fishyjoes]

I am also no expert or chemist, but this is a known side-effect of "two part dosing" - salinity raises due to increased sodium chloride.
You can lower the salinity again with RO/DI water, but each time you do that the total makeup of your water becomes more and more tilted toward sodium chloride.
You end up with relatively lower amounts of everything else so at some point you should add back in all that other stuff.

 

[dwest]

I try to combat this effect by using as much kalkwasser as possible (no sodium or chloride ions) and doing water changes.

 

[saltyhog]

I am also no expert or chemist, but this is a known side-effect of "two part dosing" - salinity raises due to increased sodium chloride.
You can lower the salinity again with RO/DI water, but each time you do that the total makeup of your water becomes more and more tilted toward sodium chloride.
You end up with relatively lower amounts of everything else so at some point you should add back in all that other stuff.

I'm doing Moonshiner's so I am adding back all the trace elements.

 

[Randy Holmes-Farley Verified Community Expert]

Folks are correct that the residual sodium and chloride is boosting salinity.

The effect is roughly a 2.4% rise in salinity each month when dosing 1.1 dKH per day and the balanced amount of calcium (8 ppm) per day.

A 2.4% rise takes sg = 1.0250 to 1.0256 or 35.0 ppt to 35.8 ppt. .