If you have a Fishbowl filled with water and seaweed in your inventory, then you can buy a Tiny net from him for 10gp and use this to catch a fish from the aquariums in his shop and obtain a Pet fish.
He sells the following items:
|Small fishing net||Fishing rod||Harpoon||Lobster pot|
|Fishing bait||Big fishing net||Raw shrimps||Raw sardine|
|Raw herring||Raw mackerel||Raw cod||Raw anchovies|
|Raw tuna||Raw lobster||Raw bass||Raw swordfish|