Fish that lives in inland water bodies, rivers,
streams and lakes are freshwater fish. Fish that lives in openwater in the
sea are marine fish. Marine fish are also called saltwater fish due to them
living in and requiring water with higher salt content.
Freshwater fish has very different colour and shape differences to marine fish. Generally, freshwater fish are not as brightly coloured and colourful compared to marine fish. (however that may not be a good guide for beginners as tropical freshwater fish can also be colourful while some marine fish can have dull colouration.)
The best way and the simplest way to find out if a fish is freshwater or marine is of course to ask the shop/person who is selling it! :)
If you are differentiating them in the fish shop, you can use tank temperature as a guide. Marine tanks is kept at a lower temperature compared to freshwater tanks. Besides the temperature difference, marine tanks also have a better and stronger water circulation compared to freshwater tanks.
Characteristics differences in rearing
For a beginner, definately go freshwater. If you really want to go marine, start with freshwater, learn the basics and then trade them back to the store in exchange for marine fish when you are confident.
You may say freshwater and marine fish are both fish requiring air water and food so why are freshwater fish easier to keep?
1. Freshwater fish are easier to keep than saltwater fish because in nature they live in smaller water bodies compared to their marine counterparts. When the water volume is smaller, the chemical composition of the water body and water quality changes with changes in the environment such as high tide and low tide, flood, drought and changes in seasons. On the other hand, due to the vast water volume in the sea, the water parameters are stable and never changing. As a result of this difference in the water stability in their natural habitat, freshwater fish are used to changing water parameters and are able to adapt quickly while marine fish are not. In the home aquarium, due to the small volume of water, water conditions can fluctuate fast especially in the beginning stage!
2. Freshwater fish are mostly farm bred fish that are bred and distributed to fish shops to be sold. Marine fish are usually wild caught fish. Due to first point above, marine fish are not easy to keep, let alone breed, thus it is more cost effective to catch them from the wild. Not taking into account water requirements, farm bred fish are also more used to artificial environment. Wild caught fish will be extremely stressed since they have tasted the open freedom and suddenly they are confined within glass panes and constant vibrations from your powerhead and water pump. Also, many fish that are used to schooling in large numbers can feel insecure and stressed when their school greatly reduced. Other than the group security factor, wild caught fish also have to undergo the capture and transport procedure.
3. Freshwater fish are generally less picky when it comes to food and most will eat whatever food that they can fit into their mouth. Marine fish require more specific diet and will often starve to death if not fed what they require.
4. Marine aquariums require more equipment and closer water monintoring than freshwater aquariums.