After owning abait shop for several years the only way I can tell you how to keep them clean is to clean them on a regular schedule. we had six tanks and we would clean them weekly. You can use bait saver in the tank to soften your water and remove the chlorine and other stuff that minnows don't like. But if it were me I would just buy them a bucket at a time and let someone else worry about cleaning the tanks.
Good Luck