Is It Fungus,bacteria,protozoan Or Flatworm Platyhelminthes?

[Shadi]

Since ten days one of my Oranda goldfish start scratching against tank decorations, four days later I noticed a large dark red spot on the lower caudal lobes, and since yesterday all my Oranda fish stays on the bottom of my tank, they move only when I feed them, I found transparent feces floating on the surface today morning. I also noticed skin lesions on another fish.
I changed 50% of tank water, when I raised temp to 88F ~ 31C fish start to be active and chasing each other but when reduced temp to 78F ~ 26C fish laid on the tank bottom. PH 6.8
Please advice how to treat this Disease?

 

[WILDER]

Hiya and welcome to the board.
It sounds bacterial you will need a bacterial med.
How many gallons is the tank, plus full stock list of fish in the tank.
And can you post water stats in ammonia,nitrite,nitrate,and ph.
Plus I would ask a mod to move this for you to the coldwater section, there names are in blue at the bottom of the forum, good luck.