with sand, you just swirl your gravel vac above the sand when you do a water change, and that will lift up the wastes and suck them up. you don't need to replace the sand at all. if there are no live plants to worry about uprooting, i would stir your sand well with a chopstick or other type tool every week to keep gas pockets from forming underneath. you only need about an inch of sand in the tank.
but IMO, a tank will be much easier maintained bare bottom - juvenile fish (i'm assuming your p. bass are juvies if they are still in a 50 gal) often need more frequent feedings, and keeping the tank bottom bare makes it much easier to clean out un-eaten food and poop.