I have just fitted a 12 v submersible pump into my 650L upright tank for filling up my backpack. It's so quick as It's a 1 inch outlet. It fills it in about 15 seconds max!
I had a job making the pump sit flat on the bottom of the tank as the 1 inch hose is quite hard and curls. So I stuck velcro to the bottom of the tank and velcro to the bottom of the pump. It now sits nice and tight to the bottom.
The pic below shows the pump sitting tight to the bottom. It was taken from the lid looking down into the tank
Matthew