Hello. Im in need of some urgent help. I’m new to Malawi cichlids but not new to fish keeping.

I recently changed my 120G tank from oscars to aulunocara. I ordered a group of 15 males from a breeder online. Everything was going great for the first month. They were all social, eating well, and appeared healthy. I am a good fish keeper. I do regular water changes and keep a keen eye on things. Two weeks ago I had my first death. It was out of the blue. No symptoms no warning. Came home to a dead fish in the corner. I thought this was very weird but all the other fish were acting normal and so I didn’t pay it much attention. I did my normal routine of a water change and all was well. Around this time I noticed one of the larger more aggressive juveniles was starting to have a pretty bloated belly. I reached out to my local community and got some advice. I ended up doing a salt water bath from him. Didn’t notice much change so not wanting to lose more fish I set up my hospital tank (I keep a sponge filter in my sump) and put him in there with metronidazole. I did this thinking it was bloat. He is doing well now just under a week later. Belly is less swollen but still not eating.

However since this bloating incident I have lost three other fish randomly. No bloat to any of them. Only sign was maybe a little isolation the day before they passed. I Noticed this in one of them last night so I pulled him to put in hospital tank and he was dead this morning. First fish in there is still fine.

I am at a dead loss. I have no idea what is affecting my colony and I’m starting to get very very frustrated. Please if anyone could help I would be really appreciative.