Sorry to hear of your losses, Jackie and Stacy. Alot of people don't understand attachments to pets and even more don't think fish are really a pet unfortunately.
Jackie, I seriously doubt he passed from any one thing unless old age or that it was anything you had or hadn't done. Four years is fairly old for our beauties. At least he went quick and didn't languish in distress.
Other aquatic creatures: ELB-guppy hybrids, Corydoras panda, Scleromystax barbatus, harlequin rasboras, mollies, nerite snails, ghost shrimp