No idea what happened! Recently added a false upside-down catfish. That was about three weeks ago. They get along well. Both were healthy. Just noticed this tonight (2/23) and did a solid water change. I took about 8-10 gallons. The tanks is 55 gallons. Water parameters are:
Nitrite 0, Ammonia 0, Nitrate 100,and pH is 7.4.
I have been doing at least 25% water changes once a week. I was on track to do a water change yesterday but I forgot. I saw this patch on my birchir's head and freaked out. Please help! I love my fish!