Just curious, why does living in an apartment deter you from having a sump?
Equipment specifically for use in AIO and Hang on back systems is extremely limited in both options and size. In the end I believe it tends to be a hardship fought by the reefer of never quite having the right tool for the right job. Just by having a sump you open up a ton of options for equipment with lots of reviews and experience from fellow reefers. Not only that, I've had AIO's before and they are better than HOB in my opinion, but I pretty much always tear the system apart and go with a sump in the end.
If you ask me, 60 breeder with a 20 gallon long sump is the perfect setup for cheap. If that's too large, go down to a 40 breeder. Both options bring a ton of equipment options to the table but also livestock options. A 60 breeder has enough space to be a forever home for a small bristletooth tang (in my opinion with the correct rockwork) which will help a ton with algae control. You'll also have enough room for many other options like small angelfish, a few wrasses, and a few other fish. While a 40 breeder is likely enough for a juvenile bristletooth, its likely too small to be a forever home to one. (I would recommend an algae blenny aka a lawnmower blenny in its place) You still have a good many of options of fish though, while going any smaller would really limit those options.
