Hey Paul,
I must respectively disagree on that last statement.
It's very possible to restrict a return pump's discharge to a point where you can damage the pump.
If restricted too much - it can get hot. If it get's too hot - it can be damaged. I did this before and totally wasted a brand new Mag12 in no time at all.
Attached is the only surviving picture of my "first" attempt at building a valve manifold for my 75 reef tank.
You can only see half of my disasterous design - imagine a tee running up from what you see - with 90 degree elbows and two more ball valves ( not full port ball valves - but reduced port ball valves ).
I included a recirculation valve and tee as well - you can just see it in the picture.
As I said - this is the only photo of this contraption I have now.
Both the manifold and the photos are gone now.
I toasted the Mag12 - thermal shutoff after a few minutes of use.
Then it would cool - and like an idiot - I turned it back on and repeated this unfortunate cycle enough to fry the pump.
Not until I tested the pump in a large container of water - did I notice the "smell" of cooked electric parts.
Just wanted to toss in my bad experience - and what I learned form it
