A 20 gallon is a bit small and limits you to your design. Simply it will take away a lot of space and you will really only have 15 or so gallons of swimming space. Anyway to incorporate a hob filter you can either hide it or somewhat build around. Get some thin strofoam sheets from home depot. Cut it to the size of your tank. Draw on where you want your rock pattern then take a steak knife and carve out the rocks. Once you finish that you can either use a hair dryer to smooth out the cuts you have made or if your happy skip this step. Now if you feel you want to hide your filter intake you can cut out a channel on the back with an intake hole on the bottom. Do this by adding second layer pieces on the back. These pieces you will be siliconing to your tank so keep that in mind. Then you will need some cement. Make a thin mix, so make it watery close to milk or water thickness. Do an entire layer of that. Let dry 24 hours. Once it's dry make a thicker mix this time. Apply a layer of that mix. Let dry 24 hours. You can use colored drylock to basically paint your background or add cement colouring to your mix. You won't need paints but krylon latex paint is safe. To silicone go to Rona or home depot get Ge silicone I. It's safe once you let it cure for a couple days. I'm about to make my next background. Will post pics.