You shouldn't get an arowana and then ship it to someone when it gets too large for you to handle. The bigger the fish, the rougher shipping is on them. I wouldn't buy a 20" black arowana off the internet, unless the guy gave a refund when the dead fish showed up to my house.
On a side point, they do grow extremely slowly. Mine is getting close to that size, I barely notice him growing at all these days.
On a side point, they do grow extremely slowly. Mine is getting close to that size, I barely notice him growing at all these days.

