He/she does look really bad. And that is probably a water change issue, built up and leading to an internal infection.
If you have to change 35 gallons every couple of days for a few weeks, that's the price of saving the fish. It may be too far gone. You wouldn't have to continue at that rate. If you have 75 gallons, and if the fish recovers, you could do 30 gallons weekly then, if the fish is alone in the 75. It could actually become well enough to be able to build up its fat reserves and have a bulbous head again. Right now, I'll wager that whatever is hitting it is is making it use all its resources to stay alive.
If you have a standard flowerhorn set up, then its a bare tank with no gravel or plants. The nitrogen cycle in such tanks is weak, and that pushes extra work onto the fishkeeper. Flowerhorns are big fish, and big waste producers. You can see 3 gallons a day isn't cutting it from looking at your fish. You're doing about 20 gallons a week, and look at the fish. It needs help beyond that.
I'd consider getting a python water change system to make it easier. With buckets, you and your flowerhorn won't be friends for long, although you'll have great upper body strength. Big fish are work.
I'd also explore using some salt, although I don't know your water or what concentrations would be best. Melafix is weak stuff, and the skin of the fish looks irritated. A short salt treatment might help. Check online for dosages.