I looked up liters to gallons, and if I'm right, 20 gallons is 75 liters. Wow, that's big for a BP! If the sides are high, you might consider getting something with lower sides. I house my females in 50 liters and my males in 30 liters. The floorspace is the same in each, but the sides are only 9" and 7". I've had larger cages in the past, and they seem to like smaller better. BTW, if you're in the market for a good cage, I recommend the product Really Useful Boxes. Hope that helps.