Clown fish issue

[The Grim Reefer]

A couple months ago one of the Ocellaris (SP?) clown fish that had been in my tank about a month started swimming disoriented, with it's head up, and a few hours later died. Now, about 2 months later the other one I bought at that time is doing the same thing. Just happened out of the blue. Is this some kind of known disease?

I added a new mate for this one about 2 weeks ago thinking after 2 months since the other had died all was clear. Guess I should have asked before adding the new clown....