Now that ur water is good. Don't do anything, let the water run, cycle and no water change or chemicals. Try starving him for few days if his not eating. Flowerhorn are hardy fish, they will eventually adopt to ur water.
I don't agree with this advice at all.
We know the tank is not cycled, and the op is not posting numbers but rather stating the water is good. Even if the op did massive water change there is no way 100% of the ammonia and or nitrites would be removed.
Any amount is harmful. The suggestion I would max is keep doing large water changes daily but avoid cleaning the filter as we want to allow the colony of beneficial bacteria to develop. Also, in order to feed the bacteria we need a food source. The fish also requires food to keep well and be able to fight off infections or parasites. Feeding every 2-3 days with small amounts will definitely help.
During this time when the tank is not fully cycled the fish is definitely stressed. Doing everything we can to minimize this is the goal.
Dim lighting, lots of plants and hides aka cover, proper temps, quality food, and most importantly a steady flow of clean water free of water and chlorine.