[madhippoz]

Do you have other fish in your tank? Most Brackish fish can acclimate to freshwater, but it doesn't usually work as well in reverse. Many true freshwater fish may not do well at all in a Brackish tank.

As for how easy it is, buy the hydrometer as suggested, you just have to make sure that when you do water changes you also add more salt. But you can't just dump the salt in, you have to have at least one bucket of water to add your salt in, put in an airstone, and let it sit for a while to help dissolve the salt. Other than that, just add and remove water as you normally would. Make sure that you are using a type of Marine Salt like that needed for salt water tanks. "Aquarium Salt" that comes in the milk cartoon is a salt tonic for freshwater, and will not make your tank Brackish.