My discus has turned dark

[kwan]

I have recently bought 3 Red Marble discus. After a week, their tails and fins have turned black. I immediately changed water a couple of times and noticed the dark fins are gradually recovering to their original colour, but is still very dark in some places, especially the tail. Please see photo.

Can someone tell me what is the problem here? I have checked my water and my ph, kh, gh, seems fine. Ammonia and nitrite are zero, natrate is low. I am not sure if they some kind of disease.

[chuck]

IMO you have nothing to worry about, Discus fins are usually normally dark colored. Expecally in Pigeon blood strains. (look at marlboro reds, tangerines etc). if the fins are not frayed or have cotton like growths on them I would not be concerned.
Things to be concerned about would be if the body turns dark in color for more then one day , the fish stays to itself, becomes lethargic not eating. white stringy feces, breathing out of one gil scratching......
if you feel better you can use Kosher salt (aquarium Salt) at 1 tablespoon/gal for a few days. but if you have live plants take them out cause the salt will kill them at that dose.
hth
chuck

Or just put them in a light-blue background.

They look pretty clean to me.

alex