A potion or scroll of invisibility.

Invis pot. There always is one if that room spawns

This is one of the puzzle rooms that will always cause a specific item to spawn an that floor. And that’s the invisibility potion like others said. That is kinda the intended way to solve it. But there are more ways:

• piranhas die on land if you move them there (blast wave, stone of blast, bombs, phase shift, curse of displacing, curse of displacement, fadeleaf, teleportatoin trap, warping trap)
• timekeeper’s hourglass (might also require swiftthistle or potion of haste if it’s not leveled high enough)
• ethereal chains (if the wall is thin and level high enough)
• teleporting there and back (stone of blink)
• blinding them (wand of prismatic light, flashbang, blindweed)
• charming them (wand of transfusion)
• terror (scroll of terror)
• amok (stone of aggression, scroll of rage)
• make them allies (wand of corruption, scroll of siren song)
• make them fall into magical sleep (scroll of lullaby, stone of deep sleep)
• not waking them to begin with
• maybe stormvine
• maybe scroll of dread to despawn them

Weeds don’t work on water. I mostly play mage and just throw an item onto the door and kill them with wands. Only things I really used was invisible potion and hourglass. Some strategies would depend on their position and how close they are to each other.

Curse of displacing especially on long range weapons like whip.

Using seeds with sandals of nature should work. Same for traps and reclaim trap.

For the original piranhas wiki says they are immune to freezing, paralyzed, rooted and gas in general. Further says that they are always twice as fast as you. But I dunno how much that would apply to ShPD.