I agree with the others. I want to be clear: People are not trying to ruffle your feathers to be a "jerk". Keeping fish is more than having something pretty to look at - it's about the care and wellbeing of the fish. A Flowerhorn should never (unless very small) be in a tank that small. That's not a jerk comment, that's the truth of what type of care that fish needs.
It is plausible that the tank size may have contributed to how this injury occurred and as your water parameters are likely less than ideal, may also be contributing to the worsening / delay of healing. Clean water will do more than most over the counter medications for this fish to heal.
If you are truly interested in caring for this fish, you'll need to do the following:
1. Upgrade your tank. This fish (to me, but maybe not others) appears stunted already. A larger tank will have a directly beneficial affect on the fish.
2. Provider us with your water parameters: Ammonia, Nitrite, Nitrate, pH
3. Provide us with your stocking: Are there other fish? What kind?
4. Provide us with the decor choices - what kind of decorations do you have?
