In order to keep the fish alive, you need to consistently feed them fish food.
Soon enough you'll gain access to upgrades for things like fish food and your laser cannon.
Tetra fish food set A selection of tetra fish food set A selection of Tetra Range fish food which ar.. .
Tetra fish food set A selection of Tetra Range fish food which ar...
A scuba diver threw some fish food to a gigantic eel and a thresher shark.