Hi Kenji,

If your fish is still behaving the same after water change it may be due to the new enviroment. Flower horn is a terrritorial type of fish. It controls its territory and when is in the new enviroment or a new territory it starts to defend itself before it take charge.
Try place 1 or 2 stones (about the size of your fish) or ornaments plus some pea size stones in your aquarium to provide some hiding place. When you do this it is important that u first take our fish.

Try this method, it will make the environment more soothing.