When you produce something from nothing, audiences see this as real magic. Show a glass of water. Place a handkerchief over the glass and, after picking up the glass and removing the handkerchief, a goldfish is now swimming in the water! Audiences applaud like crazy!
No cost involved (other than purchasing the goldfish).
No special rubber gimmick required.
Use this method to produce several goldfish, if desired, during your show.