Ranchu Black fin, Gold fish

[davidwolf]

The bottom right fin is turning black with my goldfish ranchu. I tested the tank, and the perimeters look good to me and all the other fish in the tank or thriving. He started last night staying in the bottom left side of the tank and comes in and out. He stays Most of the time by staying away from the majority of the fish. I don't see any aggression in my goldfish tank. I was wondering if y the Blackfinn causing it. Any treatment recommendations

 

[Aiken Drum]

Is it the whole fin turning black or patches and spots?

How long have you had the fish?

Please take a read through the unhealthy fish sticky and give as much detail as you are able.

https://www.aquariumadvice.com/forums/f17/before-posting-about-unhealthy-fish-read-this-32451.html

 

[m.j.gomez]

The bottom right fin is turning black with my goldfish ranchu. I tested the tank, and the perimeters look good to me and all the other fish in the tank or thriving. He started last night staying in the bottom left side of the tank and comes in and out. He stays Most of the time by staying away from the majority of the fish. I don't see any aggression in my goldfish tank. I was wondering if y the Blackfinn causing it. Any treatment recommendations

Hello, do you think you could have had an ammonia spike or mini cycle? Most of the time if goldfish get black on fins or body it's due to ammonia burns. It like a scab on a person. They get burnt then the black shows.
Another way is if fins get nipped & it starts to regrow. I would test water to make sure cycle is still intact. Maybe lighten up on feedings till you can figure what's causing the problem. Hopefully this helps you!!!!!