While we're on Mags here, I recommend EVERYONE using one for your main return buy and stock an extra impeller to have on hand. Mine cracked on me and I had to order one thru the mail. Overnight of course. Now, the powerheads kept everything alive in the main, but my sump/fuge was in jeopardy as it is filled with life (critters, macro algae, rock, etc).
I did get thru the day's wait by placing a small powerhead down there to keep the oxygen going.
But..... ya know what dummy did???
Left the heater in the fuge and the sensor in the main.
Can anyone guess what happened???? Par boiled my sump.
Anyway - extra impeller(s) ya'll!!!