Regular water changes weekly, I would try to do 100% water change. But only 20% water change a day until you get it at 100%(I learned my lesson, I did 100% and my fish would not eat properly for 4 days straight). You don't want to change the pH of the water too much, can be fatal and put stress to your fish for a long period of time. And if your worried about dropping your pH too much, then add some dead corals or keep it with crush corals sand(do some pH testing first before you jump ahead with anything dealing with corals).
Beef up or pumping up the fish. In other words, start feeding it a variety of food. A great brand of pellets(New Life Spectrum, Ching Mix, Grand Sumo, Hikari, Omega One etc). Frozen food is use as a treat probably about once a week or once every 2 week. I would try to avoid freeze dried, I don't like them at all. They can soak in water and get expand bigger, also can carry air and this can cause some issue with your fish swim bladder. It's not very common, but it has happened before. Frozen bloodworm is also ideal, but I would prefer krill myself. Some people like to keep the skin of the krill on, and some likes to take it off for an easier meal. The skin of the krill itself is suppose to act as an red color enhancement(that's what scientist believed how flamingo turned pink).
well as ur from india the easiest to find enhancers would be
XO Ocean Free Humpy Head (for kok development)
" " " " Starry (for pearl development)
" " " " Super Red Syn (for improved colouration)
u also get ching mix in a few places....dont buy the taiyo version of humpy head its not very good.....xo humpy head has shown good results on my flowerhorns in the past and most of my friends use it for their fish too