This is tricky.
If you wanted to use a HOB (Hang On Back) overflow, assuming you can find one that fits your AIO section, you could use that to go down to a sump.
FWIW, I despise HOB overflows and will never use another if I can avoid it. I would much rather drill the tank, even if I have to do it myself, than deal with that nonsense. I eventually had to fill my last one with a block of 30ppi sponge because my lettuce sea slug kept ending up in my sump, which created its own headache too. Not to mention water on the floor.
Depending on the design and layout of your AIO, you
could run a bit of tubing (with a ball valve to shut it off) from your return section, over the back of the tank, and into the sump. Priming it might be a pain though. I don’t have any great ideas off the top of my head except maybe leaving the return pump in the AIO return chamber but hooking it up to tubing that runs into the sump. Leave it off except when you've had a power outage or need to re-prime the siphon down to the sump. Then, hook the sump return pump back up to the AIO return nozzles.
The sump itself can be as simple as a second-hand aquarium with a return pump and some media. Better if you use a sump kit designed for the standard tank size you’re using, or just cut some acrylic sheets to size. You can make it as fancy or as simple as you like.
Also, have you looked into nano skimmers? The
Tunze Comline DOC series is solid. How big is your AIO compartment? Mine is 4.75" across, which leaves plenty of room for one of those or something similar. I’m actually eyeing the
IOAOI nano skimmer now.
The big question is, how much space do you have in your stand? If you’re going through the effort, you might as well put in the biggest sump that fits, since more water volume is always better.
