Teugelsi mystery death

After a few weeks of struggle my 9 inch teugelsi died. I post this now in hopes of better understanding what caused this tragedy, so that I or somebody else may save future bichirs. Now I posted about this problem a while back and all I got was pointless bickering about how my tank wasn't this or that so It'd be nice to get some helpful feedback this time around. Specs 100 gallon, built in sump and fx5 filtration, tank is borderline overstocked and will be moved into a 210 gallon soon, problems with ammonia have been present before swapping the 405 with the fx5. I will include pics and try to describe the timeline. First I noticed almost bleach like stains on the bichirs sensory spots on his face( nostrils, lips, almost to eyes), then the eyes started to cloud up at which point I decided to treat with tetracycline for 12 days in a 13 gallon tank. At this point I don't know if he had eaten or not but without a doubt he did not eat anything within the treatment days. After the treatment and moving to his old tank the bleach stains and most of the eye clouding had faded away. I then noticed along the side of his body his slime coat and skin was disintegrating. He looked confused and was allowing the prochilodus I own to suck at his side which may explain the slime coat problem. I then Moved him to treat with triple sulfa and he died shortly after. It looked like his swim bladder or neurological system was ruined at time of death. Thanks you for reading and if you have any advice it would be greatly appreciated. P.S. should I be worried about the other 7 bichirs that were in the tank with him.

First

Well the ammonia was having a few spikes because of my over feeding. I fixed all the problems with that and he was doing fine when he was in treatment except for not eating then I threw him back and that's when he started having death symptoms and I think the flagtail is fine just because he has never sucked anything else even a ray but I do understand he could be a problem and will keep a close eye on. My thinking is that he got bacterial infection I didn't treat soon enough and it either went neurological or swim bladder. That with the not eating spelled death. But I don't know if tetracycline really killed whatever it was. To me it if strange because bichirs are so hardy.

Ok thanks everybody for your help but I do need one last opinion. My delhezi looks like his gill covers are looking a little washed, almost rubbed out. He does not have any other symptoms nor does he look bleached out like the now late and great teugelsi. I am wondering if I should treat the whole tank, which would include a variety of species of fish. If I were to treat which would be the best all around least harmful bacterial medication ? I will be adding a few pics of the delhezi and teugelsi( during treatment) for examination the rest are for your viewing pleasure ;D

Synodontis angelicus, gold nugget pleco, polypterus weeksi( in background), polypterus mokelembembe, polypterus bichir lapradei, polypterus ansorgii, leichartdi arowana,

Here