It's impossible to judge whether there's anything seriously wrong with your frogs. All I can tell you is that these frogs do, very occasionally, flip over onto their backs for no apparent reason, stay motionless and then behave a little disorientated when they return the right way up. It's never been a symptom of a health problem in my experience. You say that one of the frogs has white knees but it's quite common for a leg joint to be noticeably lighter than the rest of the leg so, without seeing the frog, I'm not sure that you have anything to be concerned about.


I think you ought to be concerned about the platies grabbing them though. They probably regard the frogs as potential food which could result in a frog losing a front leg if a platie grabs hold of it.