you could probably keep 4 red belly's in there but be ready to do allot of water changes put in decor for good hiding spots for your rbly's and turn down your water temp to about 22 degrees Celsius to lower aggression make sure your water temp does not go to low (keep above 20 degrees Celsius) feed well so they will not eat each other, gravel vac your gravel about 2 times a week feed various food and i have learned from help of monster fish keepers users it is better to raise your own live foods to prevent unknown deseases put away from windows because the sunlight will cuz algea growth if or when u get algea scrube it of your tank glass get a filter that it meant for a tank for twice the size of yours because these fish will fowel your water easilly if you dont and if you want a fish that can eat up most leftovers and algea i would recommend a pleco or a bigger size well armoured catfish and you dont necciserily need a light for your tank but if u want one dim it by getting a dimmer bulb or putting somthing under your light to dim it becuase your piranhas eyes can be harmed by to bright of light and get small(fine) substrate becuse food can get stuck between large substrate and start to decay fouling your water lastly take out any uneaten food