Your going to have to find out what is going on with your tank first.
Find an LFS (local fish store) and see if they can test your water for ammonia, nitrite, and nitrates
If it is ammonia it is the most toxic to your fish and can easily kill it.
You could find some instant bacteria like cycle to put in there, and perhaps ammolock or what ever they call it to lock up the ammonia.
Last ditch would be to see if your LFS can hold your fish for you while it cycles again, if that is what it is doing.
What is now water?