What is this grey spot on my dumbo betta??

[SandyLover21]

My boy marlin was kept in a three gallon with a under the gravel filter. He was promptly moved to a 2.5 it’s a real filter after o noticed his fin looking like this. I treated with aquarium salt with no change, have noticed it and it’s been four days with no change in behavior. What in the world is this and how can I treat it?

 

[NickAu]

Looks like fin rot


Can you please answer the following questions.

How big is the tank?
Has it got a heater? What is it set to?
Has it got a filter?
How strong is the current?
Is it cycled?
How often do you change water?
How much water do you change?
Do you vacuum the substrate?
What additives or chemicals do you use? Eg De Chlorinator?
Do you use tap or bottled water? If bottled water please give us the brand.
Do you have a water test kit?
Can you tell us the readings for ammonia, nitrite and nitrate?
When you clean the tank can you tell us exactly what you do.
What do you feed him? how much? how often?
Any tank mates? What sort how many?
How long have you had the Betta?
Has he got a history of illness?
Any plants or decorations? Please give details?
Can you provide a close up photo of the fish?
Can you provide a photo of the tank?

 

[Colin_T]

can you post another picture of the fish? try not to photograph it when it is near the gravel and don't circle the area