"Something Fishy" is the third short story in More Tales to Give You Goosebumps.
In this story, the main character is Eric. He is living in an apartment with his mom and his sister. Their apartment is incredibly hot, and it doesn't even have air conditioning. Eric owns a fish tank with several fish inside of it. Late one night, he dips his finger into the fish tank for fun. Eric stirs his finger around in a very specific pattern right as the clock nearest to him turns to 10:00 PM. Immediately after the clock strikes 10:00, he realizes that he is wet. Eric shrunk down to the size of a fish!
At first Eric has fun in the water. The fish tank is the size of a pool to him. However, after a goldfish attack, he realizes that living in a fish tank is dangerous. Then, Eric's mom comes in and sees a dead goldfish. She uses a net to scoop out the fish, but she also scoops out Eric. Eric tries to climb out of the net to avoid being flushed. He manages to climb to the top of the net, but then he falls off of the net. As Eric falls, he closes his eyes. When he opens his eyes, he has returned to his normal size. Eric's mom is confused because he appeared out of nowhere.
A while later, Eric still remembers the pattern that allowed him to shrink, so he decides to do it again. While Eric is swimming around in his private pool, his sister comes in with a new fish for his tank. Before Sarah places the fish in, she says that the fish is a Siamese fighting fish, "the meanest fish on earth!"