Why do I need to check the 'salinity level' or 'gravity' of water of my saltwater aquarium?

Why do I need to check the 'salinity level' or 'gravity' of water of my saltwater aquarium?

Marc Grover (Professional Aquarist) gives expert video advice on: Why are temperature and water chemistry important for my aquarium?; What pH levels are normal for my aquarium?; How do I create the right level of oxygen in my aquarium? and more...

When you're dealing with the specific gravity of a saltwater aquarium, keep in mind that saltwater fish almost exclusively come out of the ocean. The ocean runs a specific gravity of 1.023 to 1.024 almost in every tropical sea on the planet. That's what the fish are used to. If you change that dramatically, its like pH, they just have very little time to adapt to it. Changes in the ocean happen very slowly and if a fish doesn't like a particular area for whatever reason it can leave. It can't do that in your aquarium, it is totally stuck with whatever chemistry and ecosystem you create. Therefore, it's paramount that you get the specific gravity where you want it to be, 1.020, 1.021, 1.022, somewhere in that range and then you keep it there as consistently as you can. There are tricks and ways to manipulate that for parasite control and other things, but for our discussion and all intents and purposes you really want to keep it at a certain level, and you want to control it as much as you possibly can.