I tried to mix an 7" silver with an 9" black in a 210 and well the silver was there first and he wasn't going to share!! He bit the black and started to shake him. So I had to seperate them. As I was trying to catch the new black to put him in a different tank when the silver jumped out and landed on my feet, so.........I put him in a different tank instead and kept the black in the 210. The silver died a month or to after this ordeal probably because of the stress from the tank move (210 to 55) and also he was jumping around on the ground for a good couple minutes! I don't know if you guys have exp. trying to catch an arrowana that is on the ground but trust me it is not easy! My silver jumped back up to my waist after he was on the ground, an I am 6'2"!!! Anyway over all at least I didn't lose the black arrow ($120) the silver was only about 3" when I got it so it was cheep. If I was to ever try having more than one arrow in a tank agian I would make sure that they are all black for one (less timid) and try to introduce them together, also have more then two (3 or more).