Actually all of my tanks that have, had, the skimmers are four to six feet deep, three thousand gallons each and not highly stocked. The last time it happened I had only 8 12" koi in the tank. I absolutely beleive that NO skimmer is 100% safe. it is the luck of the draw. You have been lucky.
Merry Christmas Don.
P.S. I feed my Koi as much as they want to eat as I do not beleive in starving them to keep the systems clean. I just spend more time cleaning the systems.