I would not rely on weight atop the tank; they can push the lid off still. The locks that others suggested work great; I used them when I had my snakes tanked, and I still use them for my rat tanks; not an escapee yet in the 5 years I have used these. Just make sure you get the right sized locks; ask beforehand if you can return them if they don't fit. They are a little hard to get on but once on the lid, they are easy to use!