Krill,chopped earthworm's,cricket's,shrimp,and good quality pellet food such as New Life Spectrum(has vegetables and fruit) would be good food's, but no food's will make hump larger for this is entirely genetic.
Some will develop large hump's some won't.(some females develop small hump's)
Keeping nitrAte level's low will be a bit tough in 30 gallon tank(Too small longterm) with a fish that can grow as large as the Flowerhorn. (50 percent weekly water changes). IMHO