Piranhas are freshwater and quite dangerous.
Most salt water fish will die if placed in fresh water.
Fresh water fish do have scales. There are some exceptions, like catfish, but most fresh water fish have scales.
it denpents were the fish is from the ocean or fresh water lakes
Salmon
Some do- salmon spawn in fresh water but live mostly in the ocean. Most fish live in either one or the other, not both.
This link lists the most common fresh water aquarium fish (excludes salt water fish), there are hundreds.
Most pet stores that sell fish also sell fresh and salt water fish. Read up and learn about salt water tanks because they are different to take care of than a fresh water tank.
Some marine fish can live in fresh water. But most marine fish are adapted to salt water; because of the salt in the water their body needs to do things differently, and they are used to being in salt water. If you put them in fresh water, their body can't do the same thing, therefore most of them die. For more information, see the related question.
Most sea fish have sharp teeth and most river fish have no teeth
In most cases, no. The fish will die.
Most puffer fish can be found in tropical or subtropical water. You can find some species of puffer fish in fresh water however, that is very rare.
A freshwater fish is simply a fish that lives in salt-free water, such as a lake or a pond.