Was the tap water treated before introducing the fish to the tank? I use a product called tap safe every time i clean my tank. It basically makes normal tap water safe for all fish. I had this happen to a fish once too. I bought some stuff from the pet shop that is basically like a tonic. It helps pick the fish up again. Sorry, I cant remember whats its called but Im sure your local pet shop can help there. It did the trick and the fish is still alive and well!
Just curious.....does you fish have big bulging eyes? If it does, it could be a black moor. In which case, if you lose it, which hopefully you wont, you may want to try a different one. Ive never been successful with keeping black moors no matter what I try, but thats probably just me.
One more thing, have you got a filter in your tank? It could just be the fish is suffering from a lack of oxygen. If you do have one.....does it need cleaning out? The sponges inside filters need to be cleaned once a week on average.
Hope some of this is of use :)