Tadpole Dispatching

Normally I feed tadpoles live after I’ve raised them up to a large enough size but it’s a lot of work so I ordered a bunch in the size I need. My question is what is the most humane way to euthanize a tadpole where it can still be used as a feeder?
I know tea tree oil is used on fish but that would make them unsafe to feed.
Would gradual cooling be the most humane?

Gonna update on this, fed off a couple live after not finding much of anything online about this and noticed both my turtles bit the back of the head and the tadpoles seemed to die instantly. So I tried dispatching by using the thick end of a pair of tongs and giving a quick blow to the base if the head (a little behind the eyes) hard enough to kill but not completely smash the head so it was still usable. Movement stopped immediately, pinching the tail caused no reaction, touching the eye also caused nothing. Waited awhile and even after 10 minutes still unresponsive so I continued to do so with the rest and frozen them after. I think this is the same method used on freshly butchered fish for people as well.

I’d just put them in ice trays - to keep em separated. Throw some stretch wrap over the top, and stick em in the freezer. When frozen, just repeat. And, don’t invite me over for cocktails! ;D