Antique Hydrometer

Terms we often hear that are other ways to view the "same thing" are specific gravity and density.

In a nutshell, it is the amount of dissolved solids in the water, and now most commonly referred to as a parts per thousand (ppt) measurement which measures just the salt amongst those solids.

As a measure of specific gravity (S.G.), or density, it refers to how much saltier the water is than freshwater; and it is usually about 1.021-1.025 times as salty as freshwater.

In ppt (of just the salts) it is 32 ppt to 38 ppt usually. These measures can vary with temperature, and equipment.

We make some recommendations about salinty (S.G.) on our "Curing Live Rock" page. It is very important to realize that the figures given there are for curing your live rock only, not for running a tank.

Since you will throw away curing water rather quickly, a couple times often, there is no sense in making it as salty as you can. It won't make a difference during curing to run it a bit lighter than you will your tank, just as we suggest running it cooler during curing than most tanks should be run at, if possible (to keep ox levels high). Many people think there is a perfect the one and only right salinity in which to run a tank.

Here are some expert opinions from a few books that fell off the shelf on proper salinty (as S.G. or density) to keep your tank at, which are recommended as averages to keep the most things we reefers keep:

Baensch 1.021
Prasek 1.023
Tullock 1.023
Thiel 1.024
Burgess 1.025
Moe 1.020 to 1.026

There is no "one and only" specific gravity or salinity. Beware of those who say that about any aspect of husbandry!       Remember too, all seas are not the same. The Red Sea is much saltier than any other running 1.027-1.030, so a tank for those fish should be run at a higher salinity than one for Indo-Pacific which runs 1.021-1.025 or so. The Carribean and Florida are saltier than the Indo-Pacific, if you keep a tank for those animals.

I will quote Moe who says:
"In actual practice, you needn't get all in a dither about salinity. I don't think that the salinity of a marine aquarium must be any one particular value withing a range of 28 to 36 ppt, (1.020 to 1.026). With the possible exception of certain invertebrates, marine animals and biological filters do better at salinities lower than oceanic sea water.   Lower salinities keep more oxygen in the water, allow the nitrifying bacteria to work more efficiently, and reduce the metabolic work load of the fish.   It is entirely possible to maintain a beautiful tropical marine aquarium at salinities of 20 to 25 ppt instead of 30 to 35 ppt."

He goes on to say 33 to 35 ppt is probably best as an overall average for you to keep a reef tank at. You should have some of Moe's books! (See our "Recommended Reading" page.) He was a true pioneer in this.

One point is that fish-only tanks may benefit more from lower salinity, and reef tanks (with corals, inverts, etc.) like the higher end of the scale. For all of history, the fish wholesalers at LAX run at the light end of the scale, for the reasons mentioned by Moe, and because they are either shipping out or throwing away thousands of gallons a week.   That gets expensive fast. Many retail stores run at the light side for fish, and higher for coral and invert systems too. And you thought you had a salt bill?  

In any case, all the authors are all correct. There is more than one way to skin a catfish. The salinity will not be the determining factor of your success. Other aspects of good husbandry (or wifery) will be far more important.

Keeping whatever salinity value (or temperature, etc.) you choose constant and steady is much more important than which you choose. I have kept tanks everywhere on the scale, with success. A couple times I must admit, I kept tanks that were off the scale !!   


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