100 gallons, or better 125 gallons, should be acceptable for a single pacu. Bigger is obviously better though. They are large fish, but the most serious problem you may encounter is quick, jerking movements that can displace water. For that reason it is even more important to set up a tank with large dimensions, and then I would suggest leaving some space at the top. I am not sure what you are working with now or what your options are, but if you could set up a 250 or 300 then you will have ample space for a pair of pacu and room to cut off the water level. If you are limited to something in the 100-150 range, you should probably only get one. The 100 gallon / pacu rule is alright, but don't restrict yourself to it. A 400 is going to fit more than 4, a 100 may be pushing it for one, and so forth. My experience shows that they do pretty well in pairs, better in groups of three. Hope that helps some, good luck.