I'll grant that "Rainbow Fish" is a bit random for a party theme, but it was the best way I could see to combine the odd assortment of things we thought up for my daughters's joint birthday party.
Cakes and Cukes)
an Ocean Punchbowl (once upon the time there were fish in there, but they sank and were never seen again).
Fish Straws (these gummy fish worked great for the cake and straws)
jumping through a rainbow day)
easy method I wrote about this summer. The googly eyes were affixed with Jewel It glue. Each guest got to color their fish with regular markers. Every time they go through the wash, the hats can be colored all over again.