People often mean the fastest way when they say the best way. Fish imo should never be grown to maximize the speed at which they grow. Such methods encourages foods and conditions which, while adding growth, is harmful to the long term health of the fish.
A good way which encourages good but not necessarily the fastest growth is:
1) plenty of clean fresh water, e.g., an under stocked tank with very, very low nitrates and lots of water changes
2) plenty of healthy food
3) a low stress environment where each fish feels comfortable eating and swimming
4) proper water conditions (temperature, ammonia, nitrites, oxygen and perhaps to a lesser extent pH.)
5) any other special conditions that encourage that particular fish to eat, e.g., darkness for some fish, slow water movement for others, particular sized foods for others, surface plants for others, etc.
Looking at 3 and 4, keeping several Oscars in the same tank, especially from different broods and of different sizes may not work well for at least one of the Oscars unless the tank is very large, the food designed for each Oscar, and with many hiding places.
