i feel for you having to contemplate killing your Cory,but I would 100% opt for the flick on the head. Hold him/her in your hand like you are holding a stick,making sure it’s head is sticking out and use your index finger to flick the top of the head,like flicking a crumb off a table. Trust me when I say there’s no quicker method,that’s a fact.
Yes it’s a bit morbid to talk about,and it feels like you are planning terrible things behind his/her back,but if your wee Cory could thank you for an instant pain free death,then I’m sure it would,as would any living animal.
It’s either that or it suffers needlessly at the bottom of the tank.
Two flicks to make 100% sure,but he/she won’t feel the first flick never mind the second.