I just like to say I'm no expert but I have been keeping piranhas for a few years.
I have four 9/10" reds in a 125g tank and I do two 50% water changes a week to keep the nitrates down. Clean my filters once a month.
So I would recommend to test nitrate before your water change to determine how much you need to change. As they get bigger you will need to ramp up the water changes as there are messy eaters. At the minute I would say your water changes should be good as they are still small and your doing them often.
They need to be well feed at this age but don't over feed, give them as much as they will eat in a couple of minutes or you can see they have a round belly and lift out any uneaten food.
You will know when it's time for bigger tank because they will be more fighting as they become more territorial as they get bigger.
As long as you got a few plants etc... to break the line of sight you should be good for now.
keep a eye on your water quality and you should be fine, sounds as so your doing the right things
Great advice.
