Amount of salt for 3-5 gallons

Andy16

Andy16

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Dec 10, 2002
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I need to know how much salt I need to get the specific gravity to 1.022-1.025 in a 5 gallon bucket fillet about a little less then 3/4 of the way to the top. I tried to guess and add it but i went WAY over. I dont want to waste any salt so how much salt is about right for that amount of water? This way I can get it right everytime without going to high.
 
It depends on the brand of salt you are using. They can be different strengths. I would start with 1 cup and adjust from there. I use Deep Ocean Salts and I use about 2 cups per 5 gal mix to make a SG of 1.023.
 
Thanks guys, I appreciate the help. I ran out of RO/DI water to replace the salty water with so I had to use tap and it made me really mad.
 
Seeking permission for hijack

Andy16, may I (slightly) hijack your thread? Not a true hijack, really, but a relevant extension to your original question.

The question of how much salt to add to relatively small amounts of water has come up before, and your thread seems a logical place to put a question/idea that I've had on the subject.
 
Call out the chemists, again

This question is to generalize the rule for mixing small batches of SW. Since more than a few of us run 29 gallon tanks, even a 20% water change is less than 6 gallons. For the folks with true nano-reefs, mixing a whole 5 gallon box of Instant Ocean would be massive overkill. Besides, buying salt in those tiny quantities is a lot more expensive.

So:

What exactly does a SG of 1.023 mean? It means that 1 liter of the solution weighs 1.023 kilograms. It is obvious, I hope, that this was actually a rhetorical intro to the true question which follows immediately ;)

Presuming the intrepid aquarist had access to a laboratory balance, could such a one simply add 23 grams of salt to 1000 grams of water and - presto - have a nice batch of replacement water at a SG of 1.023 with no guessing. Am I way off on this somehow?

Here to learn :confused:
 
I'm going to try to be as careful as possible in answering this, since I'll easily confuse myself.

Specific gravity is a measure of density. It compares the density of whatever you're measuring to water. So distilled water would have an SG of 1.000.

Increasing salinity does increase density, trouble is, it's not 1 to 1. For example, the average salinity of seawater is 35, but this only changes the density to 1025kg/m^3, or an SG of 1.025.

You can play around with the UNESCO Equation of State, although it's probably messier than most aquarists would like to get into.

This may help. Dan Kelley has an EoS calculator:

http://www.phys.ocean.dal.ca/~kelley/seawater/density.html

You can figure out the salinity you want, and then you're set. The salinity that you get is in parts per thousand (PPT) which translates to grams per litre. I'll leave the litre to gallon conversion to you if you so desire, but I'm guessing it's about 4g/gallon per g/L.

This should allow you to dose smaller batches from bulk salt. To get to more conenient units (like cups/tbs, etc) you just need the g/cc density of your sea salt.
 
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Interesting. So let me see if I get it: if DEmich were to add 35 g of his salt mix to about 750 ml distilled water, stirred for a bit, then topped the container off at 1L, he'd be close?

I'm an empirical guy, personally. I add 1.7 cups of IO salt to 2.5 gallons of water (both of the measuring devices not being horribly accurate), and it's about where I want it.
 
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