its always best to maximize the water changes if you want fast growth.
You can either do large water changes once a week or a series of smaller ones a few times a week. Its really up to you, just make sure you get them done if you want fast growth. A high protein diet usually makes fish grow fast, but the water changes are more important IMO.
Try to get some cichlid pellets if you can't get flowerhorn food. I would try Hikari cichlid gold which comes in about 4-5 different sizes and isn't too pricey until they grow a bit and you can find some amazone, haifeng, or chingmix. But you are varying the diet which is perfect. Some freeze dried krill wouldn't hurt either (if they can fit it in their mouths).
looking nice.