No crabs cannot live in a tropical tank. If you go to a petstore they will try to sell you small fiddler crabs, however they do not live in freshwater, they live in brackish water like most small crabs. All brackish water crabs need to be able to be out of water for a certain period of time. If you do buy one of these crabs and put them in your tank they will only be able to survive around a month.
Salt water and brackish water are different in terms how the amount of salt. You can say salt water is brackish, since brackish water and salt water both contain salt. However, salt water has more salt than brackish water, since brackish water is a mix between salt water and fresh water (so it has less salt).
you can put fish in the tank but you can also put frogs in it too!
all you have to do is buy salt from a pet store ( petco recommended ) and put 2 tablespoons in per gallon.
A dragonfish commonly known as a violet goby or dragon goby is a brackish water fish. Also, they are a very peaceful & shy fish and as such should not be housed with anything but other peaceful fish (mollies, guppies or platys)
You can put it on a table if you choose. NEVER put a tank on the floor.
you can put a betta in a 5 gallon tank
No. Unless you put it in a tank with sand.
It's not often that you will need to move a fish from freshwater into brackish water but it does happen, ie when bumblebee gobies or mollies have been kept in fresh water. The best thing to do is to put the fish in a quarantine tank and gradually increase the salt level until it matches the level in the destination tank. Fish should be provided with hiding places during this time and if it is not possible to run a cycled filter on the tank, treat with ammo-lock or similar. Do not do water changes if you can avoid it, because fluctuations in salinity at this time are not a good thing - use ammo-lock (or nitrate resin if you have a cycled filter). Add salt slowly - about 1/2 teaspoon/gallon/day. Dissolve the salt fully in water and add it over a few minutes. When the salinity of the tank matches the salinity of the tank you intend to put the fish in, you can move it across presuming the quarantine period has been long enough. Remember it is not possible to do this with all species. Species that can be successfully adapted to brackish water include all common livebearers, many cichlids and most gobies and gudgeons.
you can put a betta in a 54 gallon tank
No. Do not put unleaded gas in a diesel tank.
you can put two girl bettas in one tank but not the males