How big was the dead fish and how big is your tank? If one neon died in a 4 footer.. wouldn't affect the water quality that much. Then again.. if your fish are dying, that's a bad sign for the water quality already. Yamato are known to try to escape if conditions are unfavourable. I've seldom had yamato jump out on me... doesn't take much to please them but i've definitely heard a lot about them jumping out.