Freshwater is water you drink, comes out of the faucet, rain water, water in ponds, most rivers.
Brackish water is the water in between fresh water and the ocean..it is a mixture of the two. It has a lower concentration of salt as opposed to the ocean.
Ocean or marine environment is salt water.
These are general terms.
Most tropical fish in aquariums come from fish found in tropical environments around the world, including both freshwater and salt water species. Most like temperature of 74 to 80 degrees...but it really depends on the species
hope this helps