Malawi rock face building - help!

Hi,

I hope someone can help me with some advice!

I am building a rock wall in a 1.4 m (4 1/2 ft) tall aquarium which is only
660cm (2 1/2 ft wide) wide but is 1.65m (5 1/2 ft) long. The wall is to go
against the 1.4 m end (width about 600cm).

I started to mock this up against a wall and I have found using my original
idea of Ocean rock it is just going to be too heavy & unstable (even if I
silicon all the bits). I have put plastic fruit crates against the wall at
the bootom & along the underneath of the structure to reduce the amount of
rock but I don';t think will solve my problem.

My next thought is to use an alternative to ocean rock fro the parts of the
structure that won't be seen. I was thinking of lava rock - would that be
much lighter?

Anybody have any other suggestions?

Thanks,

Sky

ps the arangement side-on would be like this belwo where x - ocean rock &
o - is something lighter an easier to work with.

|x
|x
|ox
|ox
|oox
|oox
|ooox
|ooooox
|oooooox
|oooooooxxx
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