How do I know if it is even a disease
You need to observe symptoms and match them to known disease profiles.
Some common symptoms for common diseases:
- Scratching up against things in the tank
- Hiding
- Breathing hard
- Hanging out at the bottom or at the surface.
- Spots on skin, like white dust or salt.
- Sloughing mucus or body sores.
- ...
So far all we have on your situation is hiding and suspect tank conditions (ie cloudy water).
Unfortunately, those aren't a strong correlation for any one disease/condition....still could be a number of things.
Cloudy water is a good theory given the evidence, but you said you had both of the returns pointed at the surface. It's not THAT hard to get good aeration...to me it seems like that setup would have been pretty good. Is there a hard lid on this tank? Any other reason that aeration might be compromised? Still no better theories, so this might still be it....but questions remain! :)
Hiding is associated with LOTS of maladies, not oxygen deprivation particularly, so that does almost nothing to help us pin this down – other than confirming "something isn't right".
Stay observant.
There's no evidence to suggest a parasite
yet, but if you want to hedge that bet, consider setting up a UV filter on the display (sized for parasites, not algae...one of the smaller AquaUV units should do it) and consider adding a reef-safe anti-parasite compound like what Ruby Reef offers. Neither treatment is invasive or disruptive to any significant degree, so there's not really a downside to doing them....cheap (in the long run) insurance if you decide to go that route. It'll also clear up the remaining cloudy water issue in no time flat.