Sick Habrosus Cory In Quarantine Died

A newcomer to Thinkfish forums, I have recently been posting in the "medications" discussions about my problems with the 10 habrosus corys I have had in a quarantine tank.  They were doing fine for the first 3 weeks, then five of them died after I administered a prophylactic de-worming treatment commonly used in the US, which has praziquatal and metronidizole in it.  I had thought the med was the problem, after the remaining five seemed to be doing fine, I another sick/dying one last night.  I got him out of the QT tank ASAP, and into a separate bucket trying to save him, but he died last night. 

Now I just want to try to learn from this.  I think it might be a bacterial infection, and am considering treating the QT tank with erythromycin to hopefully save the remaining four, before they get sick.  I have two pictures of the one that died last night.  He had a distinct bulge on one side of his body, his belly is bloated and pink, and the fin right over his left gill was partly disintegrated.  Also he has a reddish gill.

If anyone has any ideas what this is, I'd appreciate feedback.  I couldn't find any pictures online that matched what I saw.  I love the little guys and it's very sad.

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the fin right over his left gill was partly disintegrated

I'm personally not too concerned about this, likely just damage, unless other fish are showing similar symptoms?

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Also he has a reddish gill.

This could be normal red or if especially red, could be a sign of ammonia in the quarantine tank, are you testing for this?

I do not think the fin and left gill was just damage.  This is the side of his body that was bloated, gill very red, and a fuzzy bit over the gill.  I'm pretty sure some kind of infection.  Doubt it was worms; didn't see anything like described, and the QT had already been through the de-worming prophylactic with metronidazole and praziquatal previously.

And yes, two of the previous cories that died had the same symptoms.  I am suspecting an external bacterial or viral infection.  Other resources have finally convinced me to treat with erythromycin.  Of course that's a risk for these fish, but they are already at risk of dying from the same thing.  Two days in, the remaining four seem healthy and active, no negative reaction to the erythromycin yet.  Will keep you posted.