My niece has been assigned an experiment in Junior High that requires an aquairum divided in two (2) using a semipermeable membrane. Knowing her uncle is an aquarium hobbiest (read that "junkie"), she called me up today to ask me where she can find such a membrane big enough for a 5-10 gal tank (her dad and her hadn't found anything in the online aqaurium sites), and to ask me to help her as her mentor in this Fall Semester-long experiment.

Does anyone here know where I could find a semipermeable mebrane-tank divider, or some such material that can be used in that capacity?

She also doesn't know yet what porosity it is suppose to be, so if you know of ones in a selection of "grades" / porosity, that would be even more wonderful.

Thank you VERY much for your help!