Oh mate. Sounds like a nightmare situation for you. I never had this happen. I have had a ray freak and fly around the tank but it settled down again quickly afterwards. Sorry I can't offer any help but I hope your ray pulls through. Best of luck.
What temp is the tank?
Ive had this happen a few times but not to this extreme. Ive often seen a dented head overnight and come to the conclusion that its some type of dehydration almost. My remedy has been temp no more then 80 degrees, salt dose, and extra airstones.
Though this may not be your problem, i would advise a full range of tests, ph swings can cause major stress, extreme nitrates, etc
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I do not have a lot of experience with Prazi, maybe someone else knows if this would help, but could you add a couple of drops of Praxi to the food to help take care of any internal parasites that might be causing a problem?
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I've injected it into worms before......
The coughing could be gill flukes. They should be wiped out with the Prazi, BUT the eggs won't be. I believe it'll take a 3-4 week treatment to make sure they're all gone......
I am with Trev, do a full range of water tests and post the numbers. Some individual fish are more sensitive than others. I really want to see the dKH reading, too low can cause big pH swings and cause things like ammonia to convert to an ion not processed by biological filtration. Keep up with the big water changes and you might be able to pull her through. Keep us posted.
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My two cents is it is stress related. I do no think it would be any kind of illness unless she was already sick. You said she was fat and happy then freaked out and went downhill which is clear signs of over stressing. I doubt it is caused by another ray or the sand even though the sand was just added. If it was another ray you would see signs of aggression towards her. So this leaves issues with the water. Is this the only system you have running? Do you have a tub that you can put her in to not only isolate her but to give her clean water that is only for her? Even if it is just a tub with an air stone that is at least 30 gallons. Once isolated you know the water can be eliminated and the stress of tank mates can be eliminated. Then you can do 50% W/C on tub or tank daily and just observe her to see if she gets better or worse.
I had a retic spaz out, curl, and die for no apparent reason once. She was chomping on some worms and my big male simply got near her and she dropped her worm, spazzed out and starting breathing super heavy. A few hours later she started curling and the next day she was dead. It almost seemed like a heart attack or stroke or something. Never had any other problems and couldn't find any similar situations online:/ I wish you best of luck...
Oh mate. Sounds like a nightmare situation for you. I never had this happen. I have had a ray freak and fly around the tank but it settled down again quickly afterwards. Sorry I can't offer any help but I hope your ray pulls through. Best of luck.
Doesn't really sound like a disease or water quality issue to happen that fast. Any chance of stray voltage?