Tank devider

plastic knitting mesh for the walls (13x22" easy to cut piece and your gravel is >2" deep)
20 page plastic report binders for the edge reinforcements (11" long, can use multiple strips)
airline suction cups to hold it all in place (sold cheap in 10 packs)

Might be over kill, but how about egg crate (lighting diffuser)
I know it's sold in 48" X 24", but lets lots of water flow though.

You could cut a piece of plexiglass or something to the size you want.

Gerald the Mouse said:

You could cut a piece of plexiglass or something to the size you want.

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But the water flow wont be able to get through plexi-glass.

James From Cali said:

But the water flow wont be able to get through plexi-glass.

Click to expand...

You could always drill small holes through the plexiglass that are large enough to let water flow, but not large enough for fish/shrimp.

Could always use filter pad media and secure with suction cups.. (yea...the big ugly blue stuff)

It depends on why you're separating it...if it's for big fish, the egg crate stuff is great. If it's for tiny critters, the other methods would be good.