thats the problem with pacu. there really is no temporary tank suitable unless its atleast 200g, and that wont even last more than a year or two. otherwise, if you upgrade by fish size, by the time the tank is established, the pacu will have outgrown it. or putting it into a 55g or something, it will just outgrow in a few months. give them some proper care, and food and they will grow as long as a 55g is wide in 6 months.