Thank you for the link. I'm not sure it's fungal, but I do think it's a bacterial infection, or a bacterial infection combined with aggression. This is what I did so far: I separated L235 (the larger fish in the tank) using divider and began treatment with broad spectrum antibiotics. So far so good.

Hi Geoff, It did look like an injury to me, but then friend pointed out that in case of aggression, most damage would be done to the tail area. In contrary, here it was a head. Besides, the only larger (slightly) specie in the tank was L235, and I don't think it's capable of aggression. I tend to ...

An update: L204 didn't make it.

Hello, I got this L204 yesterday along with two small L177. They are in the Q tank. There are almost fully grown L235, two golden zebra loaches, some red cherry shrimps and 6 theadfin rainbows in this tank. It's an established tank with wood and smooth rocks. No ammonia, no nitrites. Around 1 pm ...