How's it set up? I assume it's drilled with a overflow or is it a integrated tank with just the section in the back to work with. Is there room for a small sump?
for the Mantis tank I would keep it simple. I would fill it with suitable reef substrate for digging (aragonite IMO
) and I would have a lot of live rock in the tank. Might even consider growing some cool macro algae. I would run the biggest skimmer I could fit in the system. Add a bag of GFO
or any of the suitable organic reducers as a backup and your good to go.