found this on the net, thought it sounded like a good option maybe?
I recently read about a clever trick, which would prevent any of the above catastrophes. This ingenious aquarist used pumice stone, which is an inert volcanic rock that floats. It is also soft and very porous. To prepare the pumice, he boils it in a large pot, holding them under with a heavy piece of granite or basalt. After 10 minutes or so, he removes the pumice and drops it into a bucket of cold water, and again weights it down. Once the pumice cools, it will now sink like a feather through the air. How does this prevent toppling or possible cracking? Well, with ever so light rocks, you can pile these up high in the tank without having to worry about their pressure on the glass. And their rough surface helps hold them together. They have two other advantages. Their porous interior provides yet another niche for denitrifying bacteria to live. You could easily drill pumice with a two-inch bit if you wanted to create tunnels or caves.