pumice rock

[Demonknight]

went to a local lawn and garden place and looked at rocks, they had a pile of LARGE pumice pretty cheap and it got me to wondering if it would be safe for a tank, seems like the only way to get a truly large rock in a large tank without it breaking the bottom.

 

[Matteyecus]

Most of the time pumice stone floats so that is something to consider

 

[Demonknight]

The two things I was thinking might be issues was floating and the sharper edges, from what I've read it will at some point sink and as it ages moss/algae will soften the edges...maybe work a hole into it and help it sink.