Test your aquarium water every other day and write down your readings. You will first see ammonia levels rising. A few weeks or so later you should see the nitrite levels rising and the ammonia levels dropping. Finally, after a few more weeks you should see the nitrate levels rising and the nitrite levels dropping. When you no longer detect ammonia or nitrites !!!!!!but you can detect nitrates you can assume that it is safe to add your tropical fish. !!!!!
notice the but here that is what im talking about
http://www.fishlore.com/NitrogenCycle.htm
please read if you are unsure thank you