What is the biggest aquarium canister filter?

Let me elaborate;

When I say "biggest", what I'm referring to is the number of gallons the aquarium filter is rated for. For example, in my research, I could not find a canister filter that was rated for more than 300 gallons.

EHEIM makes one that's rated for 300 gallons, but nothing bigger.

Ultimately, I'm looking for a canister filter that's rated for 1000 gallons. Keep in mind that it's for a stock tank that's above ground. I've had problems with EHEIM's aquarium canister filters because their motors are designed to work with gravity; meaning a typical aquarium setup where the water is elevated (on a stand) and the filter sits directly below it.

My setup is a stock tank, so the filter would be sitting flush with the stock tank (both on the floor) so the motor/impeller has to be strong enough to draw the water in. I purchased a pond filter, but now I need the canister part which actually holds the filter media.