Call it a long shot, but look at practicing Jews, or Judaism in general. The meat they eat has to be prepared a certain way for it to be acceptable for them to eat it, you know it as Kosher. They dispose of the animal in a more humane way than most butchers/slaughterhouses. Quick, clean, painless butchering. I havent looked much into it, but maybe you could see about buying fish from a Jewish or Kosher market? Im sure they would make death quick, painless, and clean for fish as they do for the other animals. Not sure though, as I said, and it is a long shot, I may be completely wrong. Look into it though, as this may be one of your only options, if the only thing you are worried about is the way they are killed, and how clean/painless it is.
I respect everyones life choices and decisions, but, I dont know, not all fish, just like people, can be vegans, lol. You will need to move onto bigger/more substantial food later on though, so sooner or later something will have to give..
Though you are against eating it, it would not be for you. Its for the fish. Better to give them the nutrients and proteins they need, than to malnourish them. Also, think of it this way, you are not causing the death of the other fish to be food for your fish. Youre just the delivery person, from the freezer to the tank. I imagine there are Polys in Africa as we sit here, devouring and chomping on other live fish, at least you know what you will be feeding your Polys is already dead, you wont have to watch it die, or have any part in its death, and all youll need to worry about is if they got their fill or not come feeding time..