The salinity

The salinity is a basically the most important factor of keeping marine fish. You can't just put salt into the water, but also you can't go to the sea and take some water into the tank. You need a special instant marine water. Usually you buy a bag of it, mix it with the fresh water and than you can put it into the tank. This instant marine water consists of salt, but also of other minerals which marine fish and creatures need. You have to mix all the powder with the water, even if you don't use all because it might happen that some minerals are only in the one place of the bag and if you do not dissolve it all some important minerals may be missing in the water later.