When we lost our first fish years ago we scooped him out while Leif was sleeping and when he later inquired we told him he went to visit his fishy friends. This has never been questioned. The other day:
Leif: "Mommy, did you know that another fish packed his bag and left? Why do they keep doing this?"
NM: "What?" (Paranoid that another fish may have bit the dust and thinking I am going to need to fish it out since we have a carnivorous goldfish that really tears things up.)
Leif: "The big googly eyed one, he left!"
NM: (Somewhat relieved - that fish died MONTHS ago and yes, we have two fish still in the tank.) "Did he leave?"
Leif: "Yep, he did. I don't know why they don't stay."