Cory sick

[Daniel K]

I noticed one of my corys sitting motionless on bottom of my 55 gallon planted community tank in same place almost thought it was dead then later it moved and sat on top of this plant again rarely moving at all. Very thin tail and not eating. It has this whiteish patch on it. All my other fish neon tetras, green neon tetras, flame tetras, green neon rasboras, zebra danios and blue dream shrimp seem fine for now. I know I need to clean bottom and do water change as it's been couple weeks since last. Just hoping to be able to treat and save this fish.

PH 7.3

AMMONIA 0.5 PPM

NITRITE 0 PPM

NITRATE 5 PPM

KH 3

GH 12

Temp 77°F

Edited October 8, 2022 by Daniel K
Added some details

[Hally M.]

I see his dorsal fin is a bit torn here and there. Could be a sign of fin rot.
 

If the white spot has a cottony substance over it I would say it’s some sort of fungal infection however it seems to look like a scrape. 
 

Do a water change and keep an eye on the little guy and see if anyone else is effected. Hopefully the water change will help. If not check back and hopefully we can help your little guy. If the water change has no effect I would say move him to a quarantine tub for further observation so we can come up with a treatment. 

[Colu]

First I would do daily 50% water changes and add a double dose of prime to help detoxify any ammonia till it constantly stay at zero it's more than likely the ammonia that caused the skin damage your seeing with weight loss he could also have internal parasites so I would do a course of paracleanse in food if he's still eating if not dose the tank 

[Minanora]

Cories are very sensitive to ammonia and nitrite. I think colu has the right approach.