I'm planning to carefully put my sister's goldfish in the toilet tank.
Why would you put a live animal in the tank? Is it supposed to be a joke?
Yes, I decided to act after she made fun of me in front of family
Don't harm the goldfish because you've been embarrassed. Your sister humiliated you, not the fish, so leave it alone.
But she will feel bad that her fish died and learn her lesson.
You should not kill the fish to get back at your sister! The fish did nothing wrong to you. You should find a better way to get back at your sister.