Inside is not easy for one reason: it blocks the light that corals need. Best to do a planted tank above, behind the tank or in the sump. If you can double-deck the sump by putting the skimmer and a light above the middle chamber, or by putting a lit 20g tank above the sump to feed into it, that would be optimum. The only other thing I could think of would be barriering-off 20 g of your 90 g tank, putting a power head in that section and hoping it got enough light down there. A few small holes would let pods through. But you can figure where the mandy would camp out.