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I don't think springs will work, as they provide an opposing force, and most likely the tube would tilt to some position where the water load balances the spring force; then it'll never move again. This is a "bistable" device, so what you really want is something that is more like a zener diode: allows slow motion to the 'down' position, then doesn't release back to the "up" position until the water empties out.
EDIT: This gadget - a cabinet soft closer should work: https://www.amazon.com/Buffers-Silencer-Catches-Furniture-Hardware/dp/B07BXPRN77 It should be mounted under the front half of the tube, so it slows the tube as it falls, but does not restrict the speed with which the tube (when empty) rotates back up.