In trying to resurrect our old water tank 2 summers ago, when it was out of the trailer, I filled it with water and cleaners. It swelled up a little but not much. The plastic was pretty rigid. I think it depends on how thick and rigid the walls of your particular water tank are. Ours is set under the floor of our trailer on a 1 inch thick piece of plywood (the original), and between the floor, plywood base, and the 4 frame pieces that make up the sides, it doesn't have room to swell up much anyway. Not sure where yours would be placed. Hope this helps in some way.