I have a 125 US Gal aquarium stocked with Oscars and Pacu (details in signature). I have a Magnum 350 (350GPH) filtering it with a 20W UV light (soon to be replaced with a 9W as I need the stronger one for my pond). As long as I keep up on filter maintenance the water is crystal clear. That is along with 15-30% water changes weekly with heavy vacuuming. The webiste for the Magnum 350 says it is rated for a 55 Gal aquarium but... it seems to do the job on my 125 just fine.
I think you would be fine with the 300 GPH filter. You may decide to add an over flow filter (Penguin 330 is my recommendation, strong filters, last a good long time and very easy maintenance) at some point and / or a UV light (I just bought a 9 W where the water flows in a cork screw pattern around the light. In theory this exposes the water to the light 5 times longer than a straight flow pattern. Makes sense logically... hope it works in reality...)