You can house two arowana together, so long as they are happy with one another, or you have a huge tank. I used to have two young silvers that stayed together for about 4 months in a 135. They grew to be about 7 inches. I never had a problem with them picking on one another. I have seen in the lfs two, three and even 6 silvers up to 12" in length kept in the same tank, usually around a 10 gallon. those were usually beat up pretty bad. Size of tank is a big issue, the bigger the tank the more space they have to get away from one another. The way to get two to live together the best IMO is to grow up several 5+ and slowly take out one at a time several months apart. Until gradually you have the two that you want. but this is over at least a year, and still is not fool proof.
