Lowering salinity mixing calculation

[noobreefer2]

Hey everyone,

I’m trying to lower the salinity in my 75-gallon tank from 1.028 to 1.027 using a 10-gallon water change. I ran the numbers and calculated that my new water should have a salinity of 1.017, but I’d love for someone to double-check my math before I proceed.

Here’s the formula I used:

((Sf×Vt)=(Sc×Vc)+(St×(Vt−Vc))

Where:
Sf = 1.027 (target final salinity)
St = 1.028 (current tank salinity)
Vt = 75 gallons (total tank volume)
Vc = 10 gallons (water change volume)
Sc = ? (salinity of new water, solving for this)

Plugging in the values:
(1.027×75)=(1.028×65)+(Sc×10)
77.025=66.82+(Sc×10)

Solving for Sc:
Sc=(77.025−66.8210)/10=1.017

So, based on this, I should mix my new 10 gallons of water to 1.017 salinity to bring the whole tank down to 1.027 after the water change.

Can someone double-check my way of solving this and let me know if this looks correct? Thanks in advance!

 

[KrisReef]

(1.028×65), I think the 65 should actually be 10(VC)?

 

[exnisstech]

Seems like a lot of work to drop salinity 0.001 but I guess if calculations are stimulating it could be fun. I would probably just scoop out some water and dump some rodi in and be done but I'm a simpleton ;)

 

[Reeferbadness]

Best to raise or lower salinity slowly... so i would say going with 1.020 or so would be fine and you can do this a couple of times to get where you would like it.

 

[KStatefan]

There is a calculator for this.

I think your formulas are correct but you need to remove the 1 from the specific gravity when calculating.

 

[Randy Holmes-Farley]

In a 75 gallon tank, just remove 2.7 gallons over a few days to drop from sg = 1.028 to sg = 1.027. :)

 

[BryanM]

Seems like a lot of work to drop salinity 0.001 but I guess if calculations are stimulating it could be fun. I would probably just scoop out some water and dump some rodi in and be done but I'm a simpleton ;)

This is what I would do as well.

 

[mikst]

There is a calculator for this.

I think your formulas are correct but you need to remove the 1 from the specific gravity when calculating.

I have a hard time with this calculator. I don't want to have a schedule of water changes with specific salinity for each change. I just want to know if salinity in ppt is x and I want to reduce it to y, how much saltwater do I need to replace with fresh water?

I see Randy did a Calc with specific gravity, but I usually go off salinity. I like knowing that for each 1L of sw, 35ppt is salts, 35g of it.

So, if I have a, for example, 1000L volume of sw in an aquarium and it is 36ppt and I want to lower it to 35ppt...

36g/L* 1000L= 36kg of salt.
I want it to be 35kg. So I need to remove 1kg of salt.

Solve the other way, 1000g salt = 36g/L * xL. Solve for x. So I need to remove 27.77L of sw and replace with freshwater.

Did I do that right?

 

[Randy Holmes-Farley]

So, if I have a, for example, 1000L volume of sw in an aquarium and it is 36ppt and I want to lower it to 35ppt...

Yes.

35/36 * 1000 L = 972 L

Thus, you need to remove 1000 - 972 = 28 L

 

[mikst]

Yes.

35/36 * 1000 L = 972 L

Thus, you need to remove 1000 - 972 = 28 L

I was just about to sit down and simplify my calculations, so thanks for beating me to the punch and saving me time. Have a great day!

 

[Randy Holmes-Farley]

I was just about to sit down and simplify my calculations, so thanks for beating me to the punch and saving me time. Have a great day!

You're welcome.

Happy reefing. :)