I'm hoping to do something similar one of these days. I'm curious to see how you secure it. I've pretty much decided to put in some galvanized pipe into the floor of the pond at the outside corners to hold it in place.
55 gal plastic barrels. Got it into the pond. I used 6 barrels but it looks like 4 would have worked. Since this is not a "constant level" tank I am going to cable it to the bank so I can move it down when the water drops. Too windy today to get it to where I want to secure it.