Respectfully burying a deceased pet, providing it’s legal in your area, is a good way to show appreciation for all the hours of affectionate companionship that pet gave you. Of course, fish aren’t exactly cuddly lap creatures, but one still gets enjoyment from maintaining a well-kept aquarium and caring for their aquatic pets, watching their often amusing antics, keeping them healthy and even hand feeding them. There can develop some measure of rapport with even a fish.
Animals are gifts from God, and should be treated as such. While there are public pet cemeteries, some folks prefer burying their pets on their own private property.
Flushing down the toilet seems rude, although at that stage I’m sure the fish wouldn’t care one way or the other. It’s more how WE feel about these things, and I, for one, prefer a respectful burial to the more convenient disposal.