are you sure it's Isinglass?
other types of plastic are much more prone to "yellowing" or clouding and the fix, of one exists, can be different depending upon the material. If the plasticizers are dried causing the discoloration, the look can be somewhat improved by using one of two products - 3M 210 or Plexus. Some people have their own home remedy like Pledge which can't do any harm at this point but it's improvement, if any, will only be short-term and if it is Isinglass, will destroy the material irrevocably.What happens as plastic age with UV exposure is the plasticizers dry and cause crazing internally (similar yo what happens with Lexan over time) which simple polishing, buffing or cleaning can't cure. If the above products don't help, it's time to replace.