Do canister filters lose flow with head?

[Egon]

Okay, I think we are all on board now

 

[Dan F]

Yes, there are frictional losses through lengths of hose, but head height refers to the difference between the intake and outlet water levels. Since a canister filter has its intake and outlet in the same body of water, the head height can be considered 0.

The reason they say not to put the canister too far below the aquarium is because the water pressure at that depth can cause the canister to leak.
Imagine a hose filled with water, attached to a water balloon on one end and an aquarium at the other. As you lower the water balloon, it will increase in size and eventually explode as the pressure gets higher. Same goes for the canister.

Yep, head height is the difference between the intake and outlet, no head losses in a canister. Given the same lengths of hoses it won't make any difference in flow how far below the tank you place the filter. Shorter hoses will give a slightly higher flow because of less friction loss, but the difference is minimal as long as you aren't exceeding the length of tubing supplied.