I've just made and fitted a 3d background in my 72"x22"x30"h
I purchased two 25mm thick pink styrene sheets from my local building merchants 600mm x 2500mm
I then cut this into three so that I ended up with four bits 600 x 700 (Internal heigh of the tank)then on one of the 600 x 700 bits roughly draw some rock shapes, leaving gaps between.
Cut these out and rough up the edges.
Then using a candle I carefully melted and shaped the rocks to remove the squareness.
Then glue these onto the other 600 x 700 bit again leaving spaces between. I used adheseal adhesive to stick one to the other, expensive but very good,and black.
Leave back from the edge a bit so you can silicone around when fitted..
You can also melt and shape the back piece with the candle if required to give an uneven surface.
Now because I wanted to build in my filter pipes to the background I cut them into both the back sheet and the underside of the rocks before fixing, I then covered over the remaing visable pipe(rigid) with expanding foam, this does not go where you want it so I over did it a bit and tried to make it look a bit like a branch shape..
Then when dry and stuck I covered the whole thing in an epoxy resin (available on ebay) and as this was tacking up I then covered the whole thing in kiln dried sand (block paving sand) again from the building merchants.
Leave to set. when set lift up and all the un set sand falls of and you should be left with a sand rock background
Leave for 48hrs or so and stick to the back of your tank Or/and sides with silicone..
I decided to cover about three quarters of the back of the tank leaving a space so that it looks like the depths of the river.My tank back is painted black
Dont leave any square edges to the styrene apart from those that fit to the tank ie; bottom side and top if you have the flat braces to the top.
Ensure room temperature is 75/80 degrees when using the resin or it wont cure corectly.. silicone all the eges when sticking in the tank.
Hope that helps
Gary