silver arowana and florida gar

[master arowana]

hi fellow mfk members.I was thinking about getting a florida gar from my lfs to add to my silver arowana tank.the tank is currently 300 gallons and the dimensions are 60*40*30 {inches}.the guy at my lfs told me that i could keep them without experiencing any problems at all.could you guys tell me if it would work out or not and also tell me if i need to upgrade my tank to a bigger one when my arowana and florida gar hit full size.