I used a wood hole saw, and covered both sides with masking tape. You'll need to go slow, because going too fast will make it melt, and your drill bit can get stuck in the tank if you stop and the acrylic hardens.
I have also used a wood hole saw but I found running the bit in reverse still cuts very well but you don't run the risk of the catching and cracking the acrylic. You do need a very sharp bit if you run it in reverse or it will get too hot.
