WIth the wet dry you put in enough water in the tank to run it at a certain level. Turn the filter off and water will fill the wet/dry. You have to test it to see what water level you run at so it doesn't overflow when the power goes off. If it starts to overflow take some water out of the tank. Once you have the right level - mark the water line on the wet/dry, mine came with marker stickers. And each time you change water or add top off water you fill to that level. What's nice about this it it keeps your salinity correct also, by letting you know how much fresh water to add due to evaporation. Hope I explained this sufficiently.
We had a 16 hour power outage here a few weeks ago, gave me an excuse to go out and buy a generator. Not an option for everyone. But my tanks did not overflow.
[ April 18, 2001: Message edited by: slk3599 ]