I had 8 large red belly prianha in my 125 gallon.They were really messy and easily spooked.I found that large pieces of driftwood and lots of current settled them down a little,also,if you can get them to eat pelet food like mine did with goldfish once a week for a treat,the water parimeters stayed aot better.My largest was around 8 1/2 inches.They had a large(14 inch)common pleco as a tank mate and only nipped his fin once.I eventually sold them and went reef.