It will be necessary to read and understand the information given in the freshwater aquarium category before learning about saltwater aquaria.

Setting up a saltwater aquarium is about the same as a freshwater aquarium, until you get to the stage at which water is added. Fresh seawater is best; however, that isn't easily accessible to most of us. We can go to the pet store and buy a chemical mixture that, when dissolved in fresh water, gives the fresh water all the qualities of salt water. Or, we can make our own, by using the following ingredients: For every three gallons of fresh water, use 10 1/2 ounces pure table salt, 1 1/2 ounces magnesium chloride, 1 ounce Epsom salts, and 1/2 ounce plaster of paris. It seems safer, to me, to buy the mixture at the pet store. For example, the recipe says "pure table salt," and that it (and the other salts) can be of a "technical" grade. The table salt we buy to use on our tables is not "pure." It has stuff in it to make it easy to pour, some has iodine added, some has preservative added. I don't know if "technical grade" salts have additives or not. Pure sea salt would make more sense
to me.

The most important part of maintaining a saltwater aquarium is to have the salinity constantly at the correct level, which can be checked by keeping a hydrometer floating in the aquarium. The ideal reading (on the hydrometer) is 1025, but the water is still safe if it is somewhere between 1020 and 1030. The hydrometer needs to be checked every day, and if the reading is too low, more salt needs to be added, until the reading is correct.

It is best to make a mark on the outside of the aquarium to indicate the water level. When water evaporates from the aquarium, it has to be replaced with fresh water. The salt stuff does not evaporate, and if there is less water, the salts become more concentrated, and the fish can die. The higher concentration of salts reduces the free oxygen that must be available to the fish.

If the salinity level is wrong, the fish have only a limited period to live without the salinity being corrected, and the amount of time they have depends on the size of the aquarium.
In any case, this amount of time is measured in hours, so it is imperative that the correction be made immediately.

Of course, the aquarium has to have a thermometer. Saltwater fish live best at temperatures around 65o F
or (18o C).

Many of the plants that are available for growing in freshwater aquaria cannot live in a saltwater environment. This should be discussed with the experts at the pet store. Some of the plants that do well in salt water are enteromorpha, sea lettuce, rockweed, and Ceramium.