Just remember that the tank is merely glass. You don’t cycle the "tank" you cycle the contents. This means that your current 100 gallon substrate is "cycled". That is if you plan to use the contents from your old Glass Container. But should you stock out the 300 you may kick off another cycle, but if you keep all the sand, LR, filter media, and as much water as possible you can gradually increase you stock with no problems. Good question!