I've used 1/8in. glass before on a sump I had and didn't have problems. I found some precut pieces at Lowe's that were perfect for the 29g tank I was using as a sump. Unless you are running water at high volumn through the sump, it should be ok, especially if you glue/silicone two together. You do have to be more careful about cleaning and moving things around in the sump though. While the water flowing over/under won't break them, it's easy to bump it and cause it to crack.