This depends on the size of the tank and the amount of compressed air you have available to displace the water in the bladder that will then be pushed into the tank to raise the water level. The speed with which it goes down will depend on how fast the compressed air can escape from the bladder. Our systems have the ability to raise and lower the water as an option - except with the Z-45 where it is built into the unit. Instead of focusing on this feature we have invested a great deal of engineering design and time to make sure that our grates are flat. As a result, most users of WARDJet waterjets leave the water at 1" - 2" above the grates and cut all of their material under water.