[The old man]

For a 20 gallon tank the diffuser you have should be fine. However, you can go inline with the xp2 you have using an inline one. I have the Calaqua one from Greenleaf and it works fine using the spraybar. Just hook it up to your output tubing before it gets to the spraybar. They are a bit expensive, but take up no room in the tank and look nice. Be sure you get the size that will fit your output tubing.