Red belly pacus can get 36" to 40" in length. They can also live many years (20+ years) under the right conditions. You will be needing a tank several thousand gallons in capacity to house six grown pacu. Our two rescued pacu are at least 20", and in a few years, our 300 gallon tank will probably be inadequate for them. We are already discussing the tank build that we most likely will have to do to accommodate them should they reach 30".
Pacu are extremely sensitive to ammonia and nitrites, so make sure your water parameters are always excellent. With six pacu, to keep your nitrates at 20 ppm or less, will require frequent large water changes or a big tank. Here's a site with good pacu information: http://www.postring.net/pacu/