Is this one of the Fluval tanks that comes with the giant foam block? If so, I might suggest going with a media basket, something like this one from an Etsy seller (there's others too). This way you can run more typical medial like filter floss (I use Poly-Fil), Granular Activated Carbon (GAC) or even some extra biomedia (used in case you need to quickly cycle a hospital tank):
https://www.etsy.com/listing/188890...81yfC4K1Y_-hcKP4xw8k9d1RJn4ZS7yMaAj0_EALw_wcB
Unfortunately, you're not going to get a whole lot of fish in this tank. Remember that the display area is closer to about a ten gallon tank. And water changes will go a long way to keeping your tank in good shape. I'd suggest getting a food-grade 5 gallon bucket from your local Mega-Lo-Mart, a small circ pump and a heater. Use this to premix and heat your saltwater, and then you can safely change out a larger percentage of the tank's water without stressing the livestock.
Last, don't be afraid to try some easy soft corals, the stock light should be enough for things like Green Star Polyps (GSP), mushrooms, leather corals, Zoathids, etc.
Good luck!