The first critters I purchased for my 33gal were amano shrimp. The next morning I found one about 20ft away, all dried up. I bought some nylon(i think) mesh, used for screen doors or windows, and with tape to hold it covered all the open spaces at the back of the tank. I wanted the tank to have some air circulation to minimize overheating. I haven't had an escape since.
Well, that's not entirely true. Two zebra danio's jumped out when I forgot to close the lid. However, no more shrimp have escaped.