Porous means the degree to which water can penetrate it. The more surfaces bacteria can get on to, the more beneficial bacteria your rock can host. Since this is the 'live' part of LR, the more porous the rock is, the better it filters your water. The best rock has lots of tunnels and crannies and is light for its size. Texas holey rock is pretty dense. I've seen it in some tanks but standard LR like tonga branch, fiji, etc., will look more authentic and work better.
