A rainbow fish costume is easy, inexpensive and quick. With a little creativity, you can be sure that there won't be another costume like this, anywhere. If you have a friend dressing up with you, suggest that they go as a sport fisherman in wading boots. If this costume is for a child, have their sibling dress the part.
Check Out Your Local Hardware Store
Fabric stores can be costly. Hardware stores are a great resource for costume ideas. Visit the spray paint aisle and choose a few inexpensive cans of colourful spray paint, making sure that they work on all types of surfaces.
The flooring section has waterproof underlayer for hardwood floors. This is easy to cut and foam on one side. You can easily cut out scales and spray paint the foam side. It will make an interesting fishy texture.
Ask for old cardboard boxes; they can be cut out and taped together to make a fish-shaped costume.
Duct tape is essential for any costume.
The paint department also sells canvas for very little. If you need fabric, buy canvas and spray or hand paint it.
Visit the Craft Store
Rome could have been rebuilt in a day with a hot glue gun. If it doesn't hold together with hot glue, it wasn't ever meant to be. Make sure you get a sturdy hot glue gun and lots of glue sticks. Also, look for rivet guns and rivets, if you're using canvas and don't have an industrial sewing machine lying around. Rivet the material together and weave a rope through to pull it all together.
If You Insist on Real Fabric
Buy muslin. It's usually under £1.30 a yard and can be painted. Or use an old sheet. If you're not in a big hurry, wait for a fabric sale at a nearby retailer and buy several colours of sheer fabric. If you don't want to sew, use iron-on seam tape. Fabric glue is messy and doesn't hold well, but seam tape will stay through several washes.
Accessorise, Accessorize, Accessorize!
Paint your face blue and make scales with a face-paint crayon. Top the whole thing off with glitter. Spray your hair in rainbow colours and with glitter. Make it stick up at odd angles. Wear colourful, opaque tights and crazy shoes. Bring bubbles and blow them, creating an underwater effect. Hang pieces of net off your costume as if you narrowly escaped death. Use fishing lures or bobbers as earrings or put them in your hair.