A fish that is normally stable like this (not any other Dat, this one) Dat goes unstable when there is a change in it's environment like the removal of the Oscar. The changed stressed it so now it goes black once it is used to its environment it will go light again if it doesn't like its situation for what ever reason a fish has for that it won't. So while you can go round and round about this test the theory by putting the O back for a while and see what happens or try the blood parrot idea.