I use a gelatin rocket. I make my gelatin solution, I add it a 20oz PET soda bottle, add a carbonation cap, carb the bottle to about 6psi above what the keg is at, then use a jumper to shoot the gelatin into the gas post of the keg. See picture below:
I obviously don't normally have coke zero left in the bottle, make sure it's clean and sanitized before attempting . I also typically use one of my stainless steel carb caps from my fermzilla allrounder, so that's why I have a liquid ball lock on one end, but I'm pretty sure these red caps are gas only (at least it's always super dang hard to get a liquid ball lock on them) so if you have the plastic carb caps I recommend you use two gas quick disconnects.
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