Are they the same size? or is one bigger than the other?I have a single pair, I have seen the other clown bullying this one quite a bit, could this be the problem? I witnesses the other clown ramming it when it was on its side.
Ich could be the cause, However we will need better pics I cant tell if there is ich on the fish or its just some of the white detritis the clown is laying in. ( be best to confirm there )
That being said, if you have ich in your tank it will be on your other fish as well. In my experience tangs and angels get it faster than clowns ( who have a thicker slime coat )
Ich has other tell tale signs as well, scratching. Have you noticed this at all? You should have seen it in the days leading up to the sudden lethargic clown.
From my experience, Clowns are slower with ich, where as a tang can be utterly covered in 24 hours.
I'm currently battling an ich outbreak. ( tip kids, use DIFFERENT NETS for QT and DT )
Currently my entire stock is in a QT tank being treated for Ich. ( they were cured 2 weeks ago ) However due to the initial outbreak being in my DT, it is now going to sit fishless for 3 months to allow any ich eggs to expire. Something which I suggest you follow if you do confirm its Ich, nothing worse than doing QT for a month only to drop the fish into the DT to be infected again.