A large Plecko catfish died the other day. The smell was awful.
What are the components of such an odor? Is it ammonia?
Yuck! I’ve had a few plecos die over the years and they do smell pretty rancid after they die. Here’s the kicker - Unless the fish is small enough to flush down the toilet, I have to put it in a plastic bag and throw it in the freezer until garbage day. My apartment building has a communal trash facility in the basement and I don’t think my neighbors would appreciate my leaving a dead pleco in there for three or four days until trash day. 
While a decomposing fish will add an unhealthy amount of ammonia to your tank, I don’t think that’s what generates that godawful smell. I think it’s probably bacteria acting on the fish’s tissues.
produce?