If you want fish to grow fast I would recommend doing more water changes, perhaps 30-50% 2 or 3 times a week, or even every day if you can.
Feed them well on a variety of food but drop the beef heart. They grow rapidly on it but don't digest it properly and it can cause health issues in the long term. Feed them on fish, shrimp/ prawn, crab, mussel, or most other marine foods. Flake and pellet food is fine too. Feed them at least twice a day and try to keep them fat.
The more food you give them, the more water changes and gravel cleans you will have to do to keep the water clean.
Keep the temperature between 26-28C.
When they are at least 6 months old, but more likely 8-12months, two fish will start to hang out together. They might grab each other by the mouth and push each other back and forth. If they like each other then they will hang out in an area and chase all the other fish away from that spot. Shortly after this they usually lay some eggs. Then you have a pr and they should be moved to another tank.