Saltwater vs. Freshwater Fish

Fish cannot live in all types of water. In fact, they can have very specific needs to have a good living space. This is why it is important to test the water for temperatures, acidity (PH), nitrogen, etc. It is also important to look for parasites or disease in the tank. One thing to limit fluctuation in your water's quality is to use a larger tank. However, it is important to not have too large of a tank. One other major difference in a fish's living condition is the water's salinity.
Three main different levels of salinity for fishkeeping are:

• Freshwater
• Brackish Water
• Salt Water

Fish require different levels of salinity based off of where their ancenstors began evolving. This is becasue the fish's ancestors have maintained a similar ionic composition to the ancient body of water where its evolution begun. Freshwater contains nearly zero salinity. Salt water in the ocean has an average salinity of 35 parts per thousand (ppt). Brackish water is a mixture of freshwater and saltwater that certain species have evolved to live in. Generally, brackish water has a salinity of .5 to .35 ppt. This means that you need to moniter your fish's water salinity more for salt water or brackishwater fish. This is one of the many reasons that it is important to research your fish before buying them.