Romaurie Effect
[YT][/YT]://www.youtube.com/watch?v=p_36LhwEzFU You need to makehttpa vacuum in the upper aquarium.There are a few ways.I use a home-made vacuum pump from an old refrigeration compressor.A venturi from a water pump can be pushed to the top of the upper aquarium and it will slowly suck the air out.A vacuum cleaner with the hose reduced to a 3/8 inch diameter tube will also suck the air out but care has to be taken not to suck water into the cleaner.An air pump modified to suck the air in thru 1 1/4 inch tube will also suck the air out but without a good filter humidity will eventually burn-out the air pump.I f you're feeling really fit, you can insert a piece of hydraulic hose into the top of the upper aquarium and suck the air out.
All these methods have been tested and posted on YouTube.If you check "Romaurie Effect" on YouTube I have a number of video clips detailing how to make a vacuum pump.