Sounds like swim bladder disorder. It can be caused by a genetic disorder, injury, constipation, internal parasites, or internal bacterial infections. If it is caused by constipation, then fasting and feeding some thawed daphnia should help. Internal parasites/bacterial infections require medications. if it is caused by injury you will just have to wait it out and see if it heals.
If Edward is having a hard time swimming you should probably lower the water level so that it is easier for him to breathe (they need air from the surface) or if he is totally unable to rise maybe gently place him on a flat and smooth plant near the surface.
I would do some research on internal parasites and internal bacterial infections to see if this may be the cause. Also be on the lookout to see if he is pooping at all. Hope this helps.