Twin-wall polycarbonate.
http://www.homedepot.com/p/Sunlite-...te-Clear-Twinwall-Roof-Panel-132506/202091947
Not every depot has it, so you'll have to search your zip-code and find one nearby. I also got a quote from a greenhouse supplier/builder for a much larger sheet that was actually a better deal, but it was more material then I figured I needed for around the fish room. It is not unlike working with plywood. The best way I've found to cut it so far is a band saw (loud!!). For bigger pieces, a pulse cutting with a jigsaw worked best for me. Definitely need to wear eye protection.