how can these bastards sit there and watch the fish get into that kind of condition. I'll tell you something, if I knew the name of this curry house, they would have their own little webpage design telling everyone how bloody cruel they are the animals, then we would see how many people would want to go and eat there then.
anyway, good on you for rescuing it, for Christ's sake don't give it back to them.
Okay, the fish is suffering from severe hole in the head disease and a very bad case of fin rot. Now, whether it's beyond help is difficult to say. I would suggest that you keep the water condition in excellent condition all the time. As for the fin rot, get yourself a medication to treat this. I don't know where you are so have a look online or even better, seek advice from a vet. You may find that the fin don't grow back properly, it doesn't look too good. I think you will need some kind of medication for the hole in the head will start once again, depending on where you are in the world will govern what kind of medications are sold in that country. Tell me where you are and I will give you an idea of what you might be able to get that will help.
I've got to be perfectly honest, hole in the head disease is curable to a certain extent when it gets as bad as your fish. Like I said, check your water every day. If you can keep the nitrate levels below 10 all the time, this will really help. It will obviously mean plenty of small water changes, maybe every other day will stop I would actually suggest doing this for one week and see how the wounds fair up. If they seem to start getting better, keep up with the water changes. If it gets worse even though you are doing water changes, medication will definitely be necessary. But like I said, give it one week with nitrate levels no higher than 10. Make sure that your ammonia and nitrites zero as well, if they're not, this will just make the situation worse. I would also suggest getting good quality pellets, frozen/live shrimp, frozen fish from the fish shop, mealworms/meal worm beetles. This is all good food and should help, if only in a small way