How big is your FH? You should consider an upgrade sooner rather than later, but you might have a plan for that already, maybe your quarantining before adding into another tank?
In regards to water chanhes, 20% is only 2 gallons of water a week, that’s not going to cut it for very long, eventually (if not already) the nitrates will build up too quickly in the tank and your 20% water change isnts going to make a difference and your FH will slowly be poisoned by its own waste.
I would suggest you bump it up to 50% per week or even more depending how large your FH is...
If you dont have one already, you will need a much larger tank for your fish, I'd say 75 gallons minimum when hes an adult. Flowerhorn are large, active fish.
As for water changes, change at least 50% once a week. If you are feeding heavily, try to change 50% twice a week. Your diligence with water changes will reflect in your fishes growth and happiness.