Great article! I have bookmarked that for future ref
The lava rock is actually very light: a reason I picked that rather than anything else. I actually had to wedge some pieces together to keep them down because they're quite bouyant. Though, some other pieces are quite heavy. Also, I noticed when I move them around in the tank they're light (I can move one with a couple of fingers no probs) but when I pull one out of the tank it becomes heavy and I need one whole hand (or both). The water is exerting forces in all directions, including upwards, which is why it's bouyant also and this might lessen the load on the bottom pane some.
The room itself is capable of taking this load: here in the UK most of our houses don't have basements (only old townhouses usually), and this is all in a downstairs room. I don't know the load levels precisely though: now I've read that article I'm going to go find out!
Also, I've read that adding rock to a tank will displace some of the water. So, if your rock weighs less than the same volume/mass of water, it would actually be lighter than having just the water. How true that is, and the precise weight of lava rock/gravel vs. water, I have no idea!!!
Edit: I forgot to add; you should see I've also reduced the amount of lava rock on the side which has the highest levels of gravel, in order to balance the weight a little more too.
My main concern was the gravel rather than the lava rock...so far no cracks, leaks or explosions :S Fingers crossed... :S
Ok, a quick update: I have redesigned the tank completely. Better to be safe than sorry! I've removed one piece of lava rock + one (smaller) of the two caves, and have also removed a ton of gravel and reshaped it.
Do you think this is better? I don't think it looks as great...but it will be less load at least.