I think it kinda depends on your bio load. A little more info on the tank would be good..
What do you have circulating the water right now. Powerheads.
If you threw an aquaclear for a 20 gal. on there. It would not cost you much $$$ and it could save you some headaches. It would allow you to run carbon & Phosban media + Some mechanichal filtration IMO
If you stay on top of your water changes you can get away without a HOB filter. I'd put a HOB protein skimmer on their first. It'll help with removal of DOC's (Dissolved Organic Compounds) as well as help to oxygenate the water and gas exchange.