F151-1,
Nick had this idea a few weeks ago.
Dividing a tank...
The quickest and cheapest divider that I have ever used can be done for less than a pound.
Get yourself down to a good craft store (Hobby Craft stores up near me do the stuff), head for the cross stitch section and somewhere round there is some plastic sheeting which looks like the stuff people cross stitch in to but with slightly larger holes and made out of plastic obviously. The sheets are about a foot square, can be glued or stitched together and you can slide plastic doccument slides on to all four sides to make it rigid once cut to size, or simply put it in and let it be larger than the width of the tank and the pressure from the bending of the sheet should keep it in place (with suction cups anchoring the top/bottom if you so wish).
All for less than a quid, dependant on tank size
good idea all the same !