Sick Featherfin Catfish

[neeto]

I have had my catfish (Shadow) for about 6 yrs. I always keep tank clean, ph good, temp good.The other day he startd swimming back & forth one end to the other, 55 gal tank. This is very unusual to do this all day. Now Shadow is laying on bottom not swimming. but breathing. top of it,s back has turned white. I'm afraid it's dying. It's floats to it's side and lays there. Please help if there's anything I can do!!