As I'm starting to think through coral selection and placement, a couple of challenges;

Allelopathy
As the plan calls for very dense stocking with a variety of species, and the total water volume of the tank is only 7.5 gallons, allelopathy is a real concern. I've created areas where same-species can be located closely together while trying to maintain enough distance (and water flow) from other species to hopefully prevent chemical warfare - especially with those corals that tend to be "poor neighbors" (i.e. Acanthastrea echinata.) I also plan on running ROX 0.8 continuously in the AC20 with the hopes of minimizing problems.

Calcium / Alkalinity / Magnesium
Based on what I've learned by maintaining small tanks with lots of SPS and LPS, I don't think even frequent water changes will be sufficient to maintain stable mineral levels. I've ordered 2 BRS peristaltic dosing pumps that will be controlled by the Apex to dose Ca and ALK. Mag can be dosed manually as needed to maintain 1350. With such a small water volume, salinity creep from the 2-part dosing will likely require occasional manual removal of saltwater so the ATO can add fresh.

The biggest challenge might be finding space to "weasel" in the dosing drip tubes