Problem with sticking a 300W heater in a 30L tank is the water might be boiling by the time the thermostat actually registers the change in temperature because the heating element can heat alot faster than a smaller one. But usually it is ok.. 150W in a 2ft should be fine.
I would probably treat the fish with metro, it is quite common for discus to get these symptoms and metro usually clears it up. Probably something to watch in the future is the number of discus you have 3 is usually a bad number as the bigger 2 will both pick on the smaller one the more you have the more the aggression is spread out, sometimes it can work but others not so well maybe this is what has happened in your case.
Metro is treated one 400mg tablet per 40L of tank water temperature around 30C lots of aeration, dose daily for 4 days with a water change. If the fish is eating you can treat metro orally.
Good luck