There are commonly two types of 'regular' table salt. One is 'Iodised' the other is not. Do not use the 'Iodised' salt in fish tanks. I would advise you to use cooking salt as that is not 'Iodised'.
No. Rock salt cannot be used for marine fish. It is missing essential minerals marine fish need. You're better off buying a proper marine salt mix from a marine aquatic store, this will contain those minerals!
The proper use of aquarium salt for fresh water tanks is a good idea. The fish find it soothing. But you must follow directions carefully; don't use too much. Make sure the product you use is made for fresh water tanks, and read and follow the directions.
If it is a fresh water fish than it isn't really meant for salt water. Do some research on the breed of the fish though because some can survive in salt water.
It is not recommended to mix salt water fish and fresh water fish. It is required to separate the two and provide each with the proper tempered water.
This link lists the most common fresh water aquarium fish (excludes salt water fish), there are hundreds.
They're designed for salt water, not fresh water! Just the way they are made. You can get aquarium salt from your local pet store, but don't put any in there if you have other freshwater fish that don't need it. Or get a tropical freshwater fish like mollies instead!
Seahorses are salt water and frogs are fresh water, there are special aquariums for salt water fish type creatures.
It is about osmoregulation. Salt water fish has some level of salt in its cells, which is low in concentration than marine water and more than the freshwater. If you put a saltwater fish in fresh water, excess amount of water will enter the cells of the fish and the cells rupture. This is why salt water fish cannot survive in freshwater.
The difference is the salt concentration in the water. Fresh water aquariums mimic the environment of lakes, ponds, or rivers. However, salt water aquariums have water that is salty like oceans. Fish are either freshwater or saltwater. Do not put them in the wrong water, or they will die.
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.
Aquarium salt, not sea salt. Make sure it is freshwater salt, not marine salt or iodized. Salt in fresh water tanks can reduce stress and help in healing. Many types of fish including the live bearers and gold fish greatly benefit from it , helping to promote reproducing.
Fresh water
Zebra Danios can handle a small amount of salt in their water but it is not a smart thing to have in your fresh water tank. The reason for this is, salt can be used as a medication for some fish ailments. If there is salt already in the water, in order to dose the fish you will have to put in a measured/correct amount of salt thereby overdosing the tank and maybe killing the fish.
because saltwater fish need cold water and saltwater