Perfect for a Little Mermaid or Under the Sea party! Balloon covered ceiling + Hanging fish are great, but the scene setter walls finish the look. May want to use the marble trick so balloon knots stay toward the ceiling.
Paper Plate Aquarium Porthole. You have so many options with this you could use fish stickers, crackers, or make your own with paper. Tissue paper for seaweed or color beads and more to create their home.