I would read the directions. Most canister filters will explicitly state how far below the water line the filter will need to be. It need to be low enough that the water enters the canister filter as fast as the pump can remove it. If the water enters the tank slower than the pump removes it, the pump will have to work to pump the extra water, which potentially results in cavitations and reduced life of the pump.
I think two feet is the general minimum, but it will vary between filters.
Algae creates 75% of the world's oxygen, so really we're all just doing our parts...