What is you intent in adding something? DO you just want to tinker or are you trying to improve your tank. You did not mention the size of the display but if you are 40 gallons or less you can achieve amazing results with weekly water changes alone!!! Water changes provides your dosing, and nutrient export. Sizing a refugium is heavily discussed but the consensus is that bigger is better. How much help will a HOB fuge actually provide is debatable! Save the cash and do regular water changes. Have fun and enjoy!!