Flex molding would not work. it has to be nailed to a wall or some sort of structure. It comes in either convex or concave.
There is a plywood that bends, I don't think Lowe's would be able to get this. It is a specialty product for high end custom cabinets.
If you are wanting to make it out of 1 piece I would laminate pieces together, if you want to make it out of 3 pieces then I would suggest the front curve being made of 1/4" plywood and the sides out of 1/2" or 3/4". If you make the sides out of solid stock then you could round the corners and the would soften it to match the tank