Hi,
Based on your description, your Flowerhorn is showing serious signs of illness. Loss of appetite for 3 days, a dark body color, and a pale or sunken stomach usually indicate severe stress, an internal bacterial infection, internal parasites, or poor water quality.
Please take these steps immediately:

• Perform a 30–40% water change using dechlorinated water.
• Check the water parameters:
  • Ammonia: 0 ppm
  • Nitrite: 0 ppm
  • Nitrate: Below 20 ppm
  • Temperature: 28–30°C
• Increase aeration to maintain good oxygen levels.
• Do not force-feed the fish.
• If the fish is producing white, stringy feces, an internal parasite is a likely cause and an appropriate anti-parasitic treatment should be started.
• If there are no signs of parasites but the fish continues to darken and refuses food, a broad-spectrum antibacterial medication may be required.

To suggest the correct treatment, please reply with:

1. A clear photo of the fish (side view and top view).
2. Is the stomach sunken or swollen?
3. Is the fish breathing rapidly?
4. Is it passing any feces? If yes, what color?
5. How old is the fish and what is the tank size?

The sooner the cause is identified, the better the chances of recovery.
Regards,
Yashwant Naik