aqua_chem
Aquarium Advice Addict
Not much pressure, unless you have a pretty gnarly balloon, in which case the construction methods use to attach the balloon to the the tank will probably fail long before the pressure is built up, either by leakage or attachment failure. Your average party balloon is very contractile, and won't cause much resistance to inflation. In this case, it's essentially a constant pressure, variable volume physics problem. Essentially, as long as the balloon can inflate, it will be at room pressure inside the balloon.