Yes and no - you do need to wait for the tank to 'cycle', which can take anywhere between a couple of days to a couple of weeks, depending upon exactly what you do. 'Cycling' refers to the process of certain beneficial bacteria growing in your filter so that fish waste can be broken down before reaching toxic levels (do a google search on 'cycling aquariums' or 'nitrogen cycle' if you want a full explanation. You can speed the process up by adding products such as cycle or filterstart, or by using large amounts of water from an existing tank - all of which introduce the beneficial bacteria and allow it to establish faster. You can also buy test kits for ammonia, nitrate, and nitrate. If you test the water over the next week or to you should notice ammonia rise, then fall as nitrite rises, then the nitrite will fall as nitrate rises. Once ammonia and nitrite levels are zero, you can start slowly adding fish. You do water changes to get rid of the nitrate.