Originally Posted by AndreyT
In any canister filter, the weight of the column of water in the intake tube is always precisely counterbalanced by the the weight of the column of water in the return tube. For this reason, the net effect of water weight in the tubes is always precisely and exactly zero. I.e. gravity plays no role in moving the water through the filter whatsoever.
Canister filters are not "fed by gravity", as it is often incorrectly believed. Wet-dry filters are fed by gravity, canister filters are not. This is the defining difference between open (wet-dry filters) and sealed (canister) filters. Canister filters, by design, do not require "feeding" at all. They are based on the idea of circulating continuous volume of "weightless" water. From the principle of operation point of view, canister filters do not have "intake side" or "return side": they are perfectly symmetrical in that regard. Speaking informally, both sides of canister filter always provide exactly the same "intake pressures", which meet inside the canister and cancel each other.
This is actually the fundamental principle that justifies the very existence of canister filters. Since gravity and weight of the water plays no role at all (the water in this system is effectively "weightless"), the pumps in canister filters does not have to "lift" the water. It only has to push the water through the media. Canister filters were invented specifically to take advantage of this principle.
As long as the system is filled with water and sealed, it can be placed below the tank, above the tank, next to the tank - anywhere. The filter itself and will not be able to feel any difference. The pump load will remain constant. This is just school physics.
P.S. The reason canister filters have height-difference limitation is not the pump load, but rather seal strength. The lower you place the canister - the greater the water pressure inside the canister becomes. If the canister is placed too low below the tank, the seals will begin leaking.
I appreciate the feedback this thread is getting. I didnt expect it to turn into a debate, but hey thats cool too. lol I was under the assumption that the information above was correct, I was simply double checking before I bought anything. I have more than a couple years of aquarium experience under my belt. Most of it is with SPS reefing though, I figured the canister filter design would be similar to the idea of a closed loop design for creating flow in an aquarium. Meaning the gravity feed to the pump negates the head the pump pushes. However this obviously means this pump actively pulls water into the filter and through the canister filter, if the filter itself is level with the tank. My complete and total POS Jebao filter works this way. I figured most canister filters were this way. But then again I have very little experience with these filters. As an experiment I held my current POS filter level with the tank and it continued to run the exact same. Unplugged it and plugged it back in and had the same results.