Oh dear

Bacterial I'd say. Possibly hemorrhagic septicemia given the colouring. Sorry for the loss.

WOTS THE CAUSE?

There can be many different reasons for illness in fish, to allow FK to comment further we would need some further basic information first.

You may find it better to post in the fourm as you get more users seeing the post (hopefully more advice) but info we need as follows:

How big is the tank (litres or UK gallons)? If unsure, post measurements
What are your current readings for pH, ammonia, nitrite and nitrate? (this bit is very important)
When was the tank set up?
Did you fully cycle the tank with liquid ammonia/fish food before adding the fish (this usually takes about 4 weeks before the water is safe)
Filter brand and size?
What is the current stock, give breeds and numbers?
How often do you undertake water changes and how much?
Do you use water conditioner, to get rid of chlorine and chloramines?
Have you added any fish recently?

Is that a cory?
Sorry for your loss, looks like he got a seriously ilness