Well it depends on what you are tying to achieve - black polyurethane spray foam is by far the easiest way to cover a side or back of a glass tank.
Polyurethane foam expands while curing & you can effectively spray it straight from a disposable can onto the inside of your empty tank or spray onto a suitable piece of PVC cut to size.
I prefer to use a purpose foam gun but they cost about £35 & a tin of foam about £8, you get far more control with a gun but, it is possible to achieve good effect with a disposable can (£7)
Next up - polystyrene sheet, comes in various thickness but might only be available in 2.4 x 1.2 mts sheets.
A 100mm sheet cost about £20 however it is easy to work with & you can get some great effects.
You need to cut the poly with a craft knife, use coarse sand paper to get the design & then lightly melt with a flame gun!
This shrinks the poly & give a very realistic rock face effect but, you then need to coat the finished design with an epoxy based paint, as most paint will melt the poly.
Epoxy is expensive, I buy by the gallon which cost about £70, you would need less but possibly £20 worth.
Next option would be polyurethane sheet foam, like house insulation. Cheaper to buy because it comes in small sheets, easy to carve & can be coated with various products like G4 pond paint or colored polyurethane paints.
some folk use cement based product but, I cant help you there!
I did put a few pics here
viewtopic.php?f=20&t=12691
I think the best ones i have made have been the polystyrene ones as you can not only get great effects but by adding texture onto the wet epoxy, like black sand or other minerals, you can get even better looking results.
Just bear in mind it would be a lot easier & probably cheaper to buy a ready made backdrop!
