I'll get measurements later
Watch this same thing but I would use a 55 gallon tank for the sump. You can add tons and tons of media before the pump.
I wouldn't mix angels and an arowana. Heating get some eheim jager heaters. Substrate is all on you. Sand/gravel bare bottom anything you want.
Okay so first you have to know the dimensions of the tank. Will it fit the space? No idea it can be 6ft long or 8 feet long. Depends on the dimensions of the tank. That space looks like it won't be able to fit but measure it.
I'll check out the vid and the heater
I think I'm going with bare bottom
When I saw fill prices 300 gals I cried
Also can the Angels hang around till I rehome them or something