Here is a cheap and easy way to make tank dividers for a fraction of the cost of shop bought ones
All you need is a sheet of plastic craft canvas, available from sewing/wool shops. I get mine from Hobbycraft, 49p a sheet
4 report binders available from any stationers usually 10 in a pack for 99p
and a pair of scissors
Measure the inside of your tank and cut the report binders to fit, 2 for across the bottom and top and 2 for the sides. Then cut the plastic craft canvas to fit inside. You can sillicone the report binders into the tank for better stability, or if its temporary they will just wedge in if you measure accurately.