Other Fish Spence has Owned
Reggie died at the beginning of the episode after Spencer forgot to feed him.
Swimmy was the second fish Spencer got in that episode. He died after two days because Spencer forgot to feed him again.
Brock was Spencer's third fish. He built an automatic fish feeder to prevent him from starving, but Carly reminded him that he has to reset it every day and Spencer got depressed. Carly then brought Brock back to the pet store before Spencer could kill it.
In iWin a Date, Spencer presumably killed another goldfish when he accidentally shattered the fish bowl with a baseball.