Columbian shark as in ariopsis seemanni? Grows to 16" max and needs full salt as an adult. Must add salt starting when they hit around 6". Go fully salt by the time they are 12". If not they will die off, but these changes in salinity will kill off your other freshwater fish. I would go with another type of fish. These grow so slow that your artc would eat it before it got to any real size anyway. ARTC get huge. Zungaro Zungaro get huge and are really nasty mean. i've kept a few. Grow to around 6'+ and are pure attitude. Would be a nice cage fight to see which would win when large. You'd of course have to grow them in seperate tanks first before adding them together at a large size. You'd need a very, very, very, large (5000 gal or more) tank to house an adult artc with anything else so that it would have a chance to escape. You'd also have to keep it well fed so that it's hunger wouldn't get the best of it's tankmates. If you truely want to keep this monster dedicate the 800 to it and grow it out huge. Post up pics for everyone to see. How sure are you that you have a true artc? Many that have been posted are smaller similar species.