To kill zombies on land, you usually have to either chop their head off, burn them down or use purification magic (if it is a fantasy world).
But underwater you can't kill the zombies the same way as you do on land. Fire doesn't burn underwater and swinging a sword to cut a fish's head is nearly impossible. Harpooning them will work on larger fishes but ineffective on smaller fishes.
Details:
The fish are around 20 centimeter in length, just think of piranhas
The undead are 'dead' organisms controlled by a zombie virus (like the one on The Walking Dead, just that it only infects animals)
The question is set in a medieval world. How the populace and the government react would isn't important, what I am asking is how can someone kill those zombie fish (assume you have unlimited materials that are available on the medieval age)?