Does high jet ATO pump creates back syphon? I accidentally had back syphon from ATO happened last week, but it was more of slow trickling from the high jet output tube into sump. It took several hours until I noticed my return section got flooded with top off water. Luckily it was small trickle so didn't flood my sump instantly. My ATO has higher water line than my sump, but during the 2 month usage of high jet as ATO pump I only have one back syphon event and I didn't know what's causing it..