It is true that 3 gallons is harmless, but I dare say, if I had to do more, it wouldn't take much longer than a few minutes
. All I do is disconnect my topoff pump from the autotopoff system, throw a powerhead with a piece of hose attached to it into the sump, and plug it in. After ~3gallons has been pumped out, I throw the same powerhead into the premixed water, and plug it into my autotopoff. In a couple of minutes, whatever I pumped out of the tank has been replaced, and all I need to do is disconnect that pump and reconnect my topoff pump. Quick and painless... unless you forget to plug in that topoff pump... which I did once. Mags sure are noisy when they're sucking in air
-Richer