A protein skimmer you've got, and will certainly want to keep. If you decide to create a refugium to grow macro algae and pods, you may want to minimize other systems. Still, the two cylinders that are linked at the bottom would be perfect for a sulfur denitrator followed by an aragonite chamber. You've just got to decide if you want one, or not. Sulfur denitrators alone have a tendency to reduce the pH of their effluent. To combat/take advantage of this, you can follow the denitrator's effluent with an aragonite sand chamber. The lower pH will dissolve calcium ion making it available to your corals. In doing so, the pH will climb back up where it needs to be.
I melt powdered sulphur down and combine it with slaked and dried limestone slurry (CaCO3). I bring the sulfur up to ~300°F to melt it and slowly mix in the limestone. The final mixture consists of 2 parts of sulphur to 1 part of limestone (by volume) The sulphur I use is 100% sulphur. The limestone I use is commonly sold as #10 white. With this mixture I can keep the proportional values to support the reaction set forth by Liu & Koenig 55S + 44CaCO3 + 50NO3(-) + 18H20 + 4NH4(+) ----> 4C5H7O2N + 25N2 + 55SO4(2-) + 44Ca(2+) + 24HCO3(-) (Liu & Koenig 2002). Following this mixture with an aragonite chamber, as the two interconnected cylinders would allow, would further buffer the effluent to the proper pH, and also dissociate a few more calcium ions.
Before you set out on making a sulphur denitrator, find out more about them. There are a few links out there describing the long term observations of these systems. Also, keep an eye on the pH of the effluent. Many european systems use sulphur denitrators. With the hocus-pocus view of aquarium keeping that we seem to have, Americans tend to follow, versus innovate. Still, a cautious approach might be prudent. Find out more for yourself and see if it's a fit. Bear in mind, the autotrophic approach trades sulfates for nitrates, and there is some speculation to the chronic effects of latent sulfates.
Your refugium will have a high denitrating potential, and only use natural means. It will also supply pods and macro-algae for you, and can exist as a nursery for fry. Still the autotrophic sulphur denitrator may be able to normalize some nitrogen peaks, thereby affording a little more comfort.
As for powering such a device, you'll definitely need to pressurize the system with a powerhead or shunt some of your main pump pressure to it using a reduced "wye" fitting. Depending on the height of your overflow standpipe, you may be able to tap into some of that pressure, however overflow standpipes aren't typically flooded, and saltwater only supplies 0.455 psi per vertical foot, so it wouldn't be motivated to enter the denitrator. Your pump or a submerged powerhead would be the best bet.
