If it's drilled through the back, there should be a bulkhead nut holding it in place. If it's drilled through the bottoms, I second, third, and fourth the RTV, the Momentive stuff (check on Amazon) gives some SERIOUS hold!
yep worked great. I still can't get over how quiet this thing is. My wife told me that nothing is running, and that I have everything turned off. Then she asked when we can get fish in it...wasn't happy when I told her that i have to re-cycle the tank now.