I think you may have a couple of things working together. First of all you should have a quarantine tank that is not hooked up to your main system for new fish. That being said my guess of your secondary problems may be this. If you had a drip system that you turned down or off to medicate, than your filtration was unable to handle all the ammonia,This caused an ammonia spike which caused stress to all the rays. Then when you turned it back on the prazi was deluded and the old rays were weakened and the flukes could then take hold.