Make fish out of paper plates or plain coffee filters. Have children first decorate the plate or filter with markers. If you're using coffee filters, give children spray bottles of water so they can spritz the filters. The colors will run and look like tie-dye. When the filters are dry, draw a triangle on one side of each paper, with the edge of the paper being one side of the triangle. Have children cut the triangle out. The hole looks like the fish's mouth. They can glue the cut-out triangle onto the other side of the fish to make a tail.
Talk with children about what a healthy environment is for a fish, then give each child a shoe box. The children can paint the insides of their boxes blue or glue blue tissue paper inside to make water. Give them pebbles or sand to glue onto one side of the box to serve as the ocean floor. They can form coral and small fish out of play dough or clay, and glue them in place. Cover the front of the diorama with plastic wrap to keep objects from falling out, and glue the plastic to the sides of the box.