From the list you gave. The highest likely to be is low dissolved oxygen. I can almost certainly rule out the rest. Temp is 78.4, tank has been up and running for over 5 months. I QT all my fish due a velvet outbreak over 2 years ago and still have ptsd from it. I QT everything now.
I will have to move the mp40 closer to the surface. Thank you sir!
This could be low dissolved oxygen OR high carbon dioxide, or both at the same time. Aeration (not just water circulation) is the best way to ensure proper gas exchange. If you add a good air stone and the symptoms don't improve after 24 hours, then something else is going on.
