If you have no fish in the tank, then there is absolutely nothing to worry about. Ammonia/nitrites/nitrates will spike high during a fishless cycle, and this is completely normal and in fact desired. It is what helps the benefical bacteria to grow in the tank. Three weeks is nothing, sometimes the cycle takes longer depending upon the initial start ammount of ammonia added and the type of filtration being used.
Don't be in a rush, especially with a saltwater tank. Only when your ammonia and nitrite readings both equal zero will the tank be considered compeltely cycled. 30% water changes once a week as well as having a protien skimmer on the tank will help reduce nitrates.
What type of filtration are you running and do you have any live rock/live sand in the tank? What are your ammonia levels? How big is your tank?
Please, be patient. The more patient you are the better off your fish will be in the long run. Use this time you are waiting to do plenty of research (books, library, aquarium societies, magazines, etc) on the particular species of fish you are interested.
~~Colesea
