Hi James, I suppose there are behavioral responses to a specific stressors in your catfish species. Are your tank-mate species of the same size? It could depend from a lot of environmental issues, i.e. the size of the tank, availability of enough refugia and so on. On the other hand, the production of chemical signals by Ameiurus and Pylodictus can trigger aggression by other species. There are a good amount of scientific literature that explain how Ictaluridae (to which belong both Ameiurus and Pylodictus) produce (especially at their juvenile stage) chemical compounds that serve as antipredator signals against their adult conspecific. But the alarm pheromones produced by these catfish species seems to don't affect the behaviour of other phylogenetically distant species, and this could fit with your observations. I'm not expert enough in this field, but look at this paper, it could be the baseline for refine your search of other publications on the use of pheromones in social behavior of catfish species.

http://link.springer.com/article/10.1023/A:1024894026641#page-1

Hope to have been a little bit useful for your purposes!

Cheers.

Paolo 

Paolo:

Thanks for your answer.  Both times the afflicted specimens were larger than the aggressors.  The Pylodictis specimen was three times the size of the Plecostomus and the Ameiurus  is at least double.  Even the smaller specimens of Synodontis are bothersome to the Ameiurus.  I had not thought about the pheromones that you speak of so I will investigate that angle further.  Thanks for the reference as I will procure it from Vanderbilt university as I don't have access to it on my own.  For the time being; I am going to remove the larger Synodontis specimens that poses the biggest threat and put them in the tank that I have for the much larger Ictalurus and Ameiurus specimens and see what happens. 

Thanks again for your help.

James