You wouldn't have to worry about the others taking his food because they would be his food. This is what comes natural to him at his size and age.. Nip the fins of any fish he can get a hold of. It bring out his natural chase for food and IMO brings out more personality. Sitting pellets do little for that but are good because they are decently balanced nutrition wise. I would still feed those. The other plus to the fin thing is there would be even less load on the bio system. The waste he produces at his size is minimal. There is no need for 50% daily water changes. Introducing that much water per day I'm sure fluctuates the tank temp, the pH, an the natural bacterias that are in our fishes water (hence the fishy smell to it) new water isn't always a good thing. There are also params we don't check regularly that are fluctuating along with that. Keeping his water clean is a must for sure.. but over doing it can also be just as bad.. especially on a young fish. To much stress like that can cause him to stunt growth in every way. Even if he isn't showing signs of it right away.. down the road they will show. Like your reds and how their color is washing out already and it doesn't look like you have any over 10". They have probably been introduced to more stress then needed early in life. With my Rhoms.. I did once a week 25% W.C and once a week 5-10% wc just enough to clean up waste. I don't know any Piranaha that needs daily wc. especially that much new water at once.. I would never change more then 25% at a time. Fluctuation in the water frequently is the worse thing you can do and it is near impossible to match his water exact. Just my 2 cents man lol