How do you keep your forms from sticking to the concrete?
You need rebar, and unless you can do it all in one pour or shoot gunite, jomama and concretemasonry are gonna say, "Told ya so" 450 gallons of water weighs almost two tons. All that weight is going to be pushing down and out. The deeper the tank of same size the more pressure at the bottom sides.
If you got to Farm and Ranch Supply Store or online, you can find a 4'X8'X2' deep stock tank that holds around 470 gallons. Various sizes; shapes, round, oblong, eliptical; materials, metal, plastic, resin, fiberglass are available. Pillows, or giant water bags, are available at military surplus outlets.
Measure twice, cut once.
Look at the nail, not the hammer. Watch the fence, not the blade.
If you hook your thumb over your belt you won't hit it with the hammer or leave it layin on the saw table.