By 20L, I mean a 20 gallon long tank, not a 20 litre tank. No specs of gold on his head and body. So it's not oodinium. The only abnormal thing is his tail fin seems sort of folded. Perhaps, tail rot? It's hard to diagnose because he is small (about 1 inch).

I have 3 peppered and 3 julii corys in a 20L. No other fish are in there. Water parameters are excellent (ammo=0, nitrite=0, nitrate=10, pH=7.2, temp=75).

One peppered cory hadn't moved for more than 24 hours and didn't eat. So, I moved him to a QT yesterday. I dosed Seachem Paraguard and he ...