Here's a pic of what you have right now:
Not good. It's essentially a wet/dry unit. Thats a big nitrate factory.
Now here is a paint drawing of what you have:
And this is what I'd do with it:
Yank the bio balls out and remove the egg crate tray the bio balls were sitting on. Put a nice skimmer in this compartment.
Now you need a baffle so you can build a refugium. The height of the baffle will sorta determine the water level in the refugium, so don't go too high. You need room for the sump to fill up in the event of a power failure, but not so full that it overflows. This is easy to determine. Simply shut off your power and watch to see how full the sump gets. Thats how much water the sump needs to hold in the event of a power failure.
Build 2 baffles for the refugium. Water will just flow over the top. You want at least 4" of sand and you want a nice softball sized hunk of cheatomorpha or other macroalgae. Maybe an 8" high baffle?
The last baffle is a bubble trap to keep your return pump from sucking in bubbles and stalling out.
The baffles are cheap plexiglass. You can have them cut to size at most any hardware store for just a few bucks. Glue them in with 100% silicone rubber. Some people would use aquarium sealent because it's 100% silicone rubber. But it's too expensive for my wallet. While you are at the hardware store get a tube of silicone. Read the label. Make sure it is 100% silicone rubber. DO NOT buy any type that has "antimicrobial" or "antibactieral" additives. It should be real easy to locate a tube of just plain old 100% silicone rubber.
You can do all this in one or two days. You have enough live rock and live sand in the tank to handle the filter needs for a couple days. Don't feed the fish while you're doing this modification. They'll just poop and create waste. Put a couple powerheads in the tank for water movement. Maybe take your big wet/dry pump and put it in the tank? You need something to create surface agitation for gas exchange while you do this. So point a powerhead at the surface of the water and let it boil the water surface for oxygen.
Get your plexiglass baffles measured and cut at the hardware store.
Shut the pump down and drain the wet/dry.
Toss the bioballs in the trash.
Clean the inside of the sump with baking soda and warm water.
Rinse it with plenty of hot water in the bathtub.
Dry it out REALLY good.
Install your baffles and glue them in with a big ole goober of silicone.
Let them dry overnight...... warm sunny room.... back patio......
It might take 2 days for the silicone to dry. It'll smell like vinegar for a day or two.
Rinse it out with more warm water.
Toss some live sand in the refugium
Set your skimmer in the sump.
Put the pump in the sump.
Fire it up!!
EDIT:
The red line shows approximately where your water levels would be:
You'll have to adjust the height of the skimmer to make it work at maximum efficiency. Just set it on top of a few shims of plexiglass or maybe even a ceramic pot. If it's too high it won't even work the bubbles up high enough to get in the collection cup. If it's too low it will skim wet all the time and just be a pain in the ass. Tuning the skimmer height is critical to efficiency...... but it's not hard to do.
This is where your baffle height on the refugium is going to be critical.