Hi Tate, when I installed our poly tank, I used some small double braid rope through pad eyes that I installed to hold it down. Here is a photo.
https://picasaweb.google.com/m/viewer#photo/101945798606645671347/5214038215011392353/5292112275086410130
I set the tank on some of those plastic tiles you can get at West Marine, don't remember the name right off.
-Aaron