When keeping a marine tank, a reef tank, or a brackish tank we all know it’s important that the salinity be maintained and correct. I mention three different kinds of tank above because they can have different salinity from one another. Sometimes you will see salinity recommendations expressed differently, sometimes you see something like 32 and others 1.025. There are different ways to measure it is all and we’ll focus on one known as “specific gravity” which is expressed with that latter decimal number.
First off, we don’t add salt to fresh water.
I realize you will see various web sites that recommend this as a treatment for sick fish or parasites. That’s a different story, that’s a treatment and is meant for the short term. Once the animal is healthy you change the water again. This treatment is best done in a hospital tank and not your main display tank. The reason this treatment works is simple, salt water is deadly to things that live in freshwater. Their bodies aren’t designed to deal with it. Fungus, bacteria, and parasites that infect freshwater fish will often die (though not always) in salty water. The fish has a greater capacity to deal with this water parameter change for the short term so it doesn’t die (as long as you don’t go too high in the saltiness or leave it in salt too long).
You will see recommendations to keep some salt in your freshwater aquarium. I don’t recommend this. You will decide for yourself but I suggest you make sure you have a firm understanding of the fish you are keeping if you decide to go for it. I say this because some fish, like molly and guppy, will be just fine and actually kind of happy with this, but if you also keep most tetra this is a bad idea. This is because a guppy and a molly are actually brackish fish and not freshwater fish. A molly can be acclimated to live in a full marine environment so it’s perfectly okay with a little salt.
A brackish tank doesn’t require much salt.
A brackish tank has the lowest salinity of the three types and is normally around 1.005 to 1.010. That’s not a lot of salt really.
A marine tank gets a bit more salt.
Though of course a reef tank is a marine tank too I differentiate the two so we can understand why we would have different salinity. When I talk about a marine tank I mean a saltwater tank that just has fish and perhaps some snails, crabs, etc. I mean to point out that this tank doesn’t have corals in it. Marine fish to just fine at 1.018 – 1.022. The main reason to do this is honestly to save yourself some money on salt. By keeping the fish only tank at a lower salinity you use less salt. The fish, of course, will do just fine at the higher salinity of a reef tank but it isn’t really necessary. If you think that someday you will add corals to your tank then I recommend just keeping it at reef levels from the start so you don’t have to worry about increasing it later. That is kind of time-consuming to do. You must increase your salinity slowly so that the fish have a chance to acclimate it. So if you go from low to high just make sure to adjust it before you add corals.
A reef tank gets the most salt.
Finally the reef tank gets the most salt. I keep my tank at 1.025 all the time. The range for a reef can be 1.023 – 1.026 but I really do recommend the higher. I have done the lower and have just found that 1.25 produces the healthiest corals. When all the other parameters in the tank were good and the corals still not thriving that keyed me in that the only other thing I could change was salinity. I did, and the corals did well. So if all your other parameters are spot on and your salinity is at 1.023 with corals failing, I suggest that’s the reason. Just a suggestion of course. Every tank is different so there might be something you are missing in your tank causing the problem like electricity leaching into the water from some piece of equipment.
So, do you really need that fancy testing tool that costs an arm and a leg?
Those are the basics of salinity. My thinking on the ranges isn’t the end all be all and you will certainly find some variation in that. Don’t get excited when you find slight differences between forums, blogs, and Facebook pages. If someone says 1.016 for a fish only tank you don’t need to tell them they are wrong or come back here and tell me that so and so said it was fine. It is fine. It’s just a matter of experience or perhaps better said, experiences.
All the above said – we need a way to measure. You can’t tell what your salinity is without measuring. There are several ways to do this, even going as cheap as a hydrometer or way up to costly digital testers. I use something called a refractometer. I find it simple to use and affordable to own. It requires very little maintenance as well. I’ve been using the same one for 15 years now and it’s just now at a point where I’m considering replacing it. This is simply because over time the hinge starts to stick and it’s a pain sometimes. That’s not a major issue. Mine is made by Milwaukee Labs but you can get it from any other company.
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Prices range from $20 to $90 and you have to make sure you are buying one for your aquarium. Refractometers are used for a lot of things, like making beer. The beer one isn’t going to work for your needs.
I also don’t recommend the swing arm cheap jobs. Sticking them in the water tends to produce bubbles as they fill with water. One single bubble, even a tiny one, stuck on the swing arm will raise it up. Because of a bubble you could end up thinking you had one salinity when in fact it was much lower. It’s not worth that risk.
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Digital versions like the one shown above are great. They are exacting and can’t be messed up by a bubble or user error. The main difference is going to be cost. They cost more, a lot more. But the exactness really appeals to some people. If you have spent hundreds of dollars on corals then perhaps the peace of mind this digital read out gives you is worth it. Or perhaps if you just like gadgets and think it’s cool, then it’s worth it.
For me I have never found the need for one even though I am a gadget lover. The reef hobby can be very expensive but it can be made somewhat more affordable by picking tried and true methods over the latest gadget.
The Salinity Pen Plus from Poseidon is a new salinity measuring tool that makes it dead simple to take quick, digital measurements of the concentration of