you should *always* add something to take out chlorines and chloramines from tap water you put in the tank. avoid any other additives. pet stores will try to sell you pH adjusters, ammonia reducers, etc., but they will only cause problems. make sure the water you add is as close to the temp of the water in the tank (~78 degrees for tropicals) as possible.
if you are not doing a fishless cycle, you will need to do partial water changes during the cycle to ensure that ammonia (and later nitrite) levels don't rise to dangerous levels that will kill your fish. check the articles about the cycle for more information. if ammonia levels rise above 1, your fish will be in danger. the cycle will take some weeks, during which you need to be monitoring ammonia and nitrite levels closely.
good luck!