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.
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)?