Salinty decrease

[LegendaryCG]

Assuming no water changes, zero dosing, 0 TDS RODI water as top-off, and stony corals in the tank is it safe to assume salinty will decrease very slowly as the corals and other organisms grow?

 

[Pistondog]

Assuming no water changes, zero dosing, 0 TDS RODI water as top-off, and stony corals in the tank is it safe to assume salinty will decrease very slowly as the corals and other organisms grow?

Why would it?

 

[LegendaryCG]

Why would it?

My assumption (which very well could be wrong, which is why I'm asking someone like Randy) is that hobbiest level equipment includes trace amounts of other elements in what is summarized as "salinity". Therefore as consumption occurs salinty decreases. In other words, what we call salinty is more or less a composite measurement.

 

[Pistondog]

With no water changes or dosing some of the the traces and other elements would get used up. Calcium would get somewhat used. Then no more is available or some crucial element is depleted so the consumption stalls. The total depleted, might amount to 0.001 specific gravity, as these are measured in ppm. Most of salinity is sodium and chloride which are not consumed in substantial amounts.
I'd guess salinity would drop from 1.026 to 1.025 or so and stay there.

 

[Randy Holmes-Farley]

Assuming no water changes, zero dosing, 0 TDS RODI water as top-off, and stony corals in the tank is it safe to assume salinty will decrease very slowly as the corals and other organisms grow?

No, that is not a correct assumption.

Foods are adding lots of ions to the water, and growing organisms are using them.

While it would be possible to add more ions via food than corals are using, its not really possible to add fewer and have things keep growing long term because the growth will become limited by whatever has become depleted.

Within hobbyists measurement capability, salinity will be stable.

 

[Randy Holmes-Farley]

With no water changes or dosing some of the the traces and other elements would get used up. Calcium would get somewhat used. Then no more is available or some crucial element is depleted so the consumption stalls. The total depleted, might amount to 0.001 specific gravity, as these are measured in ppm. Most of salinity is sodium and chloride which are not consumed in substantial amounts.
I'd guess salinity would drop from 1.026 to 1.025 or so and stay there.

I think it's about 20-fold lower than that estimate of a drop in sg of 0.001.

Assuming we start at 8 dKH for alk and allow calcium carbonate to form until we hit 5 dKH (them stops due to low alk), how much did we reduce salinity?

That 3 dKH also consumes 21.4 ppm of calcium. The carbonate associated with it weighs 32.1 ppm. Total loss is 54 ppm, or 0.054 ppt.

How much is that in specific gravity?

35 ppt is about sg = 1.026400
35 ppt - .054 ppt = 34.946 ppt
34.946 ppt is about sg = 1.026359
For a drop of aboit sg = 0.00004

 

[LegendaryCG]

Interesting thanks!