Planted Tank Guru
Join Date: Jul 2010
Location: Huntington Beach, CA, USA
Part of the concern about restricting the flow is correct, part of it is not. Many peole are still using the old way of thinking about pumps even though they are talking about mag drive pumps.
When flow is restricted to older pumps which have a direct connection between the rotating part and the impeller blades, the windings on this part can begin to heat up. As they heat more current is drawn causing more heating. It quickly becomes a death spiral until the coatings on the windings melt or bearings overheat and the motor fails.
On mag drive motors, there are no windings on the rotating parts. Only a magnet. Compared to older or larger motors, the magnet can take a lot of heat before melting down. As long as there is a small amount of water coming through to do some cooling the mag drive motor is not "strained" in any way. If water flow is totally cut off, the impeller can wind up running dry and then mechanical damage like scoring or even overheating to the point that plastic begins to melt can happen but that is a pretty extreme case which should not happen often.
To me, putting the pump before the media exposes the pump to potential damage from any sand, gravel or shells picked and is much more likely to damage the impeller than the possible overheating. I like the design used for most canisters where the pump is after the media and mounted high so that there is little chance of debris killing the impeller. I find a lot of sand in the botttoms of canister filters.