No your 6x20inchx20inch will not be ok for life. Just off the top of my head, I believe that is 150 gallons, which is way too small. A RBPacu tops off at 3 foot, not 18inches. The average is 3 foot, not the largest, so they all usually top off at 3 foot. For a RBPacu to live in for life, I would say around 1000 gallons. This summer I am going to set up a 1100 gallon pond for my RBPacu and RTC. I know this size of tank is not even realistic for the average hobbyist, even the "MFK" hobbyist, so the best option if you want one is a pond-which are alot cheaper than aquaruims. A circular pond is also better, because they are not the best swimmers and with a circular pond they will be less likely to get scared and run into the side of the pond.
Edit: RBPacu grow at around 2-3 inches a month. After it gets to 15 inches, I would take him out of your tank. If you bought him at the 2-3 inch mark like most people do, then he will outgrow your aquarium in 4 months.
