The Model "G" External (formerly the Model G-modified) is a sump that allows for an external pump.  If the sump isn't drilled, an internal pump can be used of course.  The skimmer section houses the protien skimmer and bubble tower, and the large refugium allows the hobbyist to see inside easily.  Water from both zones pours into the center area, and then flows behind the refugium zone to the external pump.

The benefit of this design is the increased linear travel time that avoids issues with microbubbles.

I like this shot, mainly because of the shadow cast by the sump.

In the above image, you can see the hole for a 1.5" bulkhead. Water will exit this hole and be pumped up to the tank with an external Reeflo pump.

A few sumps had this little box mounted inside the refugium compartment, and the hole is for a pH Controller probe. A calcium reactor drips its effluent into this box, and the probe measures the pH at all times. The box overflows quietly into the refugium section, and any excess C02 will be absorbed by the macro algae growing in this section.  That was the concept, but in reality it wasn't beneficial because we need alkalinity and calcium in the reef much more than the refugium zone (with its slower flow rate).  A box can still be used but should be in the return section rather than the refugium zone.

A 1.5" stand was made for the skimmer. Triggersys skimmers run in 8" deep water, because this compartment is 9" tall. The stand will elevate it just enough to maintain efficiency, and yet not interfere with the collection cup's removal within the aquarium's stand.

Here are a few images of the sump in front of the new stand.

The sump is 48" x 20" x 16", with a 20g refugium section.

Installation

The stand was built so the back upright could be removed, allowing the sump to be installed from the back.

The sump was inserted with the foam sheet beneath, and the stand reassembled.

The skimmer fits, the return pump is in place, and plumbing is next.

Here's how one customer plumbed it in. Two drains from the tank are connected to a horizontal pipe with two points that can drain.

The right side: Tank water drains into the refugium. The ball valve controls how much flows into this compartment. Note that the elbow is installed so that water will pour out at the surface level in the refugium, with the fitting half submerged. With it not fully submerged, air can breathe out of the plumbing. The more the ball valve is closed, the less water can drain out. Any excess water will simply travel across the horizontal pipe to the other drain (into the skimmer section).

The lower plumbing fitting is set up for the return pump. An elbow inside the sump sucks water from the bottom 1" of the return section, to avoid sucking in any air. Using unions to connect the pump allows it to easily be removed later for maintenance. Even some of the drain plumbing has unions so that sections of pipe can be removed if necessary.

The drain line going into the skimmer section is set up to drain into the bubble tower. Water will pour out the base through the teeth, behind the protein skimmer (not pictured). The light is a 6500K spiral compact fluorescent bulb to light the refugium. This size refugium will need two or three bulbs to provide adequate lighting for the macro algae growing in 20g of water.