I have both my 150 and my 110 directly on the hardwood without issue. I have spilled water previously and typically it will sponge back out if it goes under. I have a view through the bottom of the 150 and can say that there is nothing wrong with the floor underneath. I had a 72 where the 110 is for 2 years and when I moved it there wasn't anything wrong with the wood. I would think that putting a liner underneath might prohibit the woods ability to evaporate any water that did get under the tank but that is just MHO.