When they start tripping the GFCI, the are pretty much destined to the landfill or for external applications if they still run "while dry"/prior to being submerged. If they trip "dry", IMHO, throw them out.
My experience w/Mags:
I use a Mag12 for mixing and pumping NSW into the aquarium after water changes so they aren't used for very long at all nor am I "rough" with the Mag. In the past 2 years, I've gone through 4-Mag12s. The first one I got used and I've broken off the top cover to seal where the cord/epoxy interface in the pump head and still trips the GFCI when submerged but not dry. The second and third I just returned as they were under mfg warranty. The fourth I have now is only 3 months old and an estimated total use of 2hrs.
In my case, even though I made a handle to handle (sounds/reads funny) the Mag pump (no stress on the cord), they eventually begin tripping again. I'm guessing it'll be 2-3 months when this Mag begins tripping. I'll be sticking with Mags as there aren't any other submersible pumps of the same flow rate but if there were, I'd give them a try.
JME/HTH
