OK...hmmm...I don't know if you are going to be able to effectively do all this with a 20g long or high. To start with, yes the pump is like a pond pump. I'd go with a Mag Drive....probably the Mag 12 which should yield about 1000gph at the height you'll be pumping to. They are submersible and are good pumps. The drawback is that, like any submersible pump, they put heat into the water so you would have to really watch your temp for a while until you were sure it wasn't going to be too much. It would probably be fine. The alternative would be to have the 20 drilled and use an external pump. To get the water from the main tank to the sump, you will have to have an overflow. This consists of a box that hangs on the inside of the main tank. The top edge of the box has notches cut on the front and sides. You also have another box that hangs on the outside of the main tank. Connecting these two is a U tube. Water flows into the inside box and, via the U tube which acts as a siphon, into the outside box. The outside box has the line going to the sump coming out of the bottom. When the water level in the tank drops below the level of the inside box, the flow stops. This is how you keep from draining the whole tank into the floor when the return pump quits. If you just put a siphon into the main tank, you'd lose a lot of water if you had a pump failure. Also, with the overflow, if the return pump starts back up (in theory) the siphon will start to flow again which will keep the main tank from running over. I'll try to put a diagram on here that illustrates the way this works, but you've seen my artwork so no promises. Anyway, as I said in the previous post, the return pump is trying to overfill the tank. The water spills over into the overflow instead of your floor.
Now...the skimmer. You will probably have about 6" of water in the sump so this kinda rules out an air driven CC skimmer. You'll need a venturi type with a pump. Same kind of pump as the return pump. The skimmer sits in the sump with it's pump submerged. As the water flows up into the skimmer, the venturi draws air through a tube and mixes it with the water. Instant bubbles. There will be an outflow line (or lines) below the collection cup where the water will spill back into the sump. If you look at the Berlin brand of skimmer, you'll see what I'm talking about. With all this stuff in the sump, I'm not sure you are going to have room for a refugium. The sump needs to have high water flow...in this case, about 1000 gph
. The refuge needs a much slower flow. Going to be hard to separate them in the amount of space we have to work with. However, you could put another tank or container (it doesn't have to be an aquarium) above the sump. You would use a powerhead to pump water from the sump up to the refugium and let it gravity return to the sump...preferably close to the return pump intake. The carbon bag just needs to be somewhere that water will have to flow through it. OK, it won't let me upload the diagram of the overflow or the sump/refuge. Maybe the server move has something to do with it. Maybe the above will make some sense.