I recommend printing this file onto cardstock. I backed the aquarium with sticky sheet cork and put a little hot glue on the backs of the fish to prevent them from scooting around.

This can be used in a couple of different ways. Two children, who are practicing 1-1 correspondence, can use as a game by taking turns rolling a die and adding fish to the aquarium. Alternatively, children who are doing addition equations, can use the fish to find the answers.