I use either Fox Farms Sledgehammer (it’s their flushing agent) or I use dry powdered yucca from Build A Soil. I’ve read where either yucca or Dr. Bronners should be used as water conditioner before mixing up feed water or water for foliar sprays. When added to water and shaken or stirred in it creates a foam head on the water, that’s the spreader sticker agent for agricultural applications. Many peeps use a few drops of Dawn dishwashing liquid for similar purposes, its a lower budget solution than yucca.
I’m a soil grower but I never actually flush my containers… not worried about salt buildup since I’m running organic or organish inputs like Down to Earth 5-4-2 it’s organic dry mix fert made up of meals like kelp meal, oyster shell meal etc