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.
this looks like a great mothers day/grandparent gift/ teacher gift idea. Lots of options with flowerpots, buckets, etc to put it in and with the types of flowers or stems to use and filling in the pail.