i lost a bristle nosed catfish once during a tank change,
i thought i had burried him under the gravel
i was so worried i stripped the tank again,i eventually found
him alive and well.the tank was a 4ft.
i have had fish die and nothing to be seen,and on rare occasion
floating skeletal remains.
i forgot to add.................
any dead fish you know of,do a water change, bodies can decompose rapidily and turn the water.