Hi,
what a bad experience! Sorry for your loss!!!
You hadn't any chance to help your big endli (the sen also) - sometimes it's horribly astonishing what bichirs can do!
I think, the endli was injured internally when he ate the sen - obviously the reason for the bad smell when opening him (the injuries had been infected and caused an internal bacterial rot). Those finlets on the back are sharp ...
I've lost two bichirs over the years: One sen that couldn't stop to eat and "exploded" internally and ... an endli after 2 days. He also slimed out in a way i'd never seen before - also an internal bacterial infection, but this one was caused by hidden Macrogyrodactylus polypterii - they had hidden in his gills and damaged the gills and the immune system.
Normally those parasites are external - normally. But i found them in his gills, in his mouth, but NOT on his body!! He had been quarantined well with Praziquantel months before and did not came in contact with new bichirs in the following time. I know my dealer very well and he has enough experience to do the right things ...
Unfortunately all my other bichirs were infected with this parasite - obviously! Except the new ones i got with the endli: my lap and my weeksi.
I treated my fish and tanks with Flubenazol and after a week there were no parasites at all ... during this time my dealer contacted me and told me that there was something wrong with his endlies. Suddenly i asked him if he could see any parasites on them - no! They did not eat any more, were sliming and showing a very lethargic behaviour.
So i sent him from my Flubenazol and he treated his endlies the way i told him. After a week all the endlies (over 20!) were ok - eating, swimming ...
He sent me a very special "thank you"-present: the Teugelsi!
Maybe ... is there the possibility of an infection from new bichirs? It's a very special situation that an external parasite hides in the gills or mouth - but i've made this curious experience!
If any other bichir is showing a strange behaviour - there is a possibility of a strange internal infection of external parasites! The best you can do in this case is to medicate all your bichirs and tanks against these specific bichir parasites!
Anne had written much about the medication - i do not know if Flubenazol is allowed in the US ... but i do not trust Praziquantel anymore. I've heard from other keepers in Germany about the possibility that the parasite came back, even after months!
I've written so much words now - but i wanted to describe all the possibilities i know about the described symtoms. Other possibilities - "classic" internal parasites to name one - you could read in the other posts to the theme.
I hope your other bichirs will be healthy and that it was a tragical accident ...
Greetings
Uwe