It maybe too far gone..
Yes, it actually has an effect on what both levamisole and metronidazole do....plus what prazuqantel does. It's a very broad spectrum de-wormer.
No. It's got to go into the water. You may try a double doze. It is quite safe. If the fish is still not well, it maybe too far gone and organs are shutting down.
Apart from that, maintain a really clean water. Increase your water changes significantly in order to increase the fish's chances significantly.