I bought two tiny clowns and a yellow goby from LFS. All 3 were in a fully cycled 10G QT. I did not dose any medication. All three were fine for a week. one clown had a tiny nick behind its eyes. Last night It was breathing really fast and had a bulged eye[didn't look cloudy at all] on the same side as the nick. It did take food but also had a long thread like poop hanging from it for hours. It was not swimming as fast as the other Clown but other than that it looked OK until 3am. But this morning, it was already dead. What could it be and do I have to treat the other two fish for anything now?
TIA
TIA