I've found a few dried up fish on the floor over the years, though I must say, I think a badly decomposed fish in the water is a lot more disgusting.
Not so long ago I did a water change in my 180 gal. when I was very drunk. I lied down and fell a sleep while the tank was filling. I awoke to the sound of a water fall. The water had made it into my daughters room, which is right beside the tanks, though not too much of a disaster as it's in the basement. Vacumed with a shop vac and the rug dried in a couple days. Anyways, many days later, I kept smelling something 'fishy' and found my red tailed shark, all dried up amongst my dirty laundry. I guess I learned my lesson, not to do a water change unless your sober enough
A couple years back, my almost 30 years old 180 gal. fiberglass tank sprung a leak, and I put all the fish in totes, temporarily. I was out of town for almost a week and I came back home to a horrible stench. One of the totes had gone 'bad' and turned into a rotten stinking dead fish soup in the middle of summer. And this tote contained my almost 14" male Tilapia zilli ---- big fish don't smell nice when they start to rot.
Many years ago, one summer, I had other interests, and little interest in my aquariums. I through in pellets now and then, and paid little attention to my tanks. Sort of forgot about my 15 year old marbled goby which didn't need to be fed every day since it is so inactive, but had to be fed using a tube close to it's mouth in order for it to get food. And since it sits there like a big log, never noticed it had died, for quite some time. A big rotting fish has got to be one of the most disgusting things. I guesstimate the fish to have been 18- 20", though I can't say for sure, as the fish was simply too disgusting to put a tape measure to it.