Over the years we've had a number of different pets, from goldfish to hamsters, mice and gerbils, all the way through to cats and dogs, and when they pass away, depending on the animal depends on what we do with them. The fish have been laid to rest in the usual way, down the toilet and while it may seem a bit of a sad way to go, I'm not actually sure what else you're supposed to do with a dead goldfish!
The cats and dogs we've had that have died, we've had them cremated and the ashes scattered in a suitable place. They become part of the family as more often than not they're with you for a number of years, so in my opinion our faithful little friends deserve to be given a decent send off.