could I keep a datnoid in a 300gal?
Yeah that should not be a problem. But keep in mind you cant always go by gallons. The footprint is important to some fish. Say rays for an example. Bigger is always better but the footprint is whats most important. You can have like my growout tank thats 150x60x60cm wich is 540L thats perfect for growing out raypups. But would be better if it where 100x100x54cm instead. Same liter but diffrent footprint.