They grow as fast as any monster fish, mine grew a foot in 1 year from the size of 2 inchs, but I fed them like 3 times or more each day. I had them in a 20 gallon long till they were 9 inchs each and they were happy as long as I fed them, but yea your going to need about 55 or bigger to start off with, and a pond to be honest is the best place you can really put a pacu. I have 4 and there all in my 1,000 gallon outdoor pond and loving every second of it. If you have pacu and you plan to keep them for a long time a pond should be something you may want to keep in mind, if you have any questions feel free to PM me about pond ideas and I could help you out.