Sorry i havnt been online to answer your question, hopefully it wasnt too late....
As you can probably see in the attached pictures, its just a hamster bottle and a vinyl tube to replace the metal one. i split the end of the tube that is in the bottle beceause i was getting issues with bubbles getting trapped in the round tube. that wouldnt be a problem with a larger size tube or bottle. the tube is mounted submerged just below the water line so that when the water level drops the water spills from the tube and into the tank. its similar to the way when you put your finger over the end of a straw the water is held inside. thats where the inspiration came from....
the hardest part of building one is making sure the bottle is airtight, if air gets in a vacuum cant be created and the contents will simply spill into the tank. it doesnt even have to be a bottle, a hole drilled into any other airtight contsainer would work just as well. the bottle should also be built of a rigid material, mine is glass but rigid plastic will do as well. but once its together and working, it CANT fail, no moving parts, just physics doing the heavy lifting for you
i hope that was of assistance, ill try and be on more often...i plan on updating with pics of the tank here pretty soon.....maybe tomorrow, its come a long way