Hello, this is my first post on mfk. I purchased a 5" silver arowana yesterday and put him in with my 7" diameter motoro stingray and the arowana has been acting very strange. He is not swimming all the time. When I go near the tank, he will just hide and stay still on the bottom of the tank. What could be causing this? I tried to feed him and he hasn't eaten. I have blackwater in the tank to try to help out too.