methylene blue plays havoc on your beneficial bacteria colony, as well as plants and inverts so using it in the main tank can still be detrimental in the long run especially when we don't know the actual problem.
the methylene blue can be added to the tank, yes. If you want you can fill a bucket or smaller tank and use 2x the dosage of methylene blue and give the flowerhorn a dip bath in it. Yes O2 means oxygen. If hes breathing heavy and without knowing anything else I'm going to say your tank is either crashed or there isnt enough dissolved oxygen in the water. Follow the steps I laid out and see if he doesnt improve a little. The methylene blue will help him with respiration as will the salt. It will also help against any bacterial or fungal infections but right now I am still convinced its a water quality issue.