Do not feed beef heart. You are correct about them not being able to digest it properly. It's a terrible food source, It might yield fast growth but also results in failing health and short life span .
Meal worms, or other insect diet is ideal for channa and yields a good growth rate. A quality pellet food is also good , personally I like to use both with insects as the main staple.
The more varriety the better. Watch carefully not to over feed especially during the winter months. The subtropical Channa are prone to obesity and they'll gladly eat themselves to death if allowed too !