a 10g probably, a 20g maybe, But I've found out the hard way of course anything 30g or up tends to bust pretty easy when not level or supported properly. I would def. change that asap before your luck runs out. I've used a piece of plywood under a 40g on a fireplace (the bricks werent perfectly level) and I used shimms under the plywood to even it out, after several months the plywood bowed just enough to pop the seal, luckily that time though I caught it halfway drained and was able to save my fish