Do canister filters have to be at the SAME level of the tanks or can the canister filter be lower- like on the floor below the tank?
Below is fine. Most people keep them in the cabinet below an aquarium. : )
Its fine below the tank. Petco has a special right now on the Penn plax 1000 for $67, great deal!
https://www.petco.com/shop/en/petcostore/product/penn-plax-cascade-1000-canister-filters