Why not do it above, on a shelf say, as a display refugium. Sandbed, crabs, shrimp etc. along with the macroalgae for nutrient export. The benefits are you can pump from the 29 gallon up to the 3 gallon and let gravity return water to the 28 gallon. No pods going through return pumps, and another "display tank" for you friends and family to oooo and ahhhh at.