Make the fish out of cardboard or felt, and thread it to the button. For the aquarium, you just need a box. Remove all 8 flaps, then cut windows in the sides and slots for the fish in the top with a box cutter or exacto knife.
Another of July project our Pre-k class did. This was a hit with all the kids. Some turned out better then others but overall the craft was a success. Toilet Paper Roll Fireworks Stamp Craft for Kids - Great for a of July art project!