Where'd the Orange Fishy Go?

Yesterday Carter and I ran into Petsmart to pick out a new fishy. We currently had two, but one had been belly up for a while. WELL, when we got back from the pet store, the "belly up" fish seemed to have recovered. So, we put the ailing fish into his own temporary home to rest while the new orange fish was released into our tank.
This morning when we woke up, the ailing fish seemed to be doing a lot better, but Carter was asking the question, "Where'd the orange fishy go?". The orange fish was nowhere to be found. I was scared to look around in fear that he had jumped out and was laying on the counter all crusty. Well, he wasn't on the counter, but he was on the floor. Ewwwwwwwwwwww. We are so upset. We can't figure out how in the heck our new fishy got out of the tank since there is a secure lid. I guess it will remain one of life's many mysteries. Poor orange fishy, rest in peace little fella'.