Yuri,
I'm going to give you a recommendation, give you a rudimentary reason for that recommendation, then give you some background info on why some writings done by hobbyists (even if they are doctors) might not always be the best advice. Since this is the New to Reefkeeping Forum, I don't want to get too complicated.
I would not put a DSB in a 24g Aquapod. I would recommend a shallow sand bed if you like the look of sand. The additional water that would be displaced by the sand is a whole lot more useful to you than having a DSB for denitrification. Water changes in a nano are quite easy and effective at removing nitrates. The bacteria don't have eyes....they don't have a clue if they are in the ocean, a 750 gallon tank, or a 24 gallon nano. In other words, in spite of what you've read, the bacteria will do what they are programmed to do and you'll get denitrification. You've apparently been doing a LOT of research and I applaud you for that. The remainder of this post will explain why a lot of the DSB articles are confusing to newbies and long time hobbyists alike.
Dr. Shimek has long said that a DSB is not useful in a nano tank because he is not merely interested in getting denitrification, he's interested in setting up mesocosms. A mesocosm would be considered an entirely self-contained ecosystem with the full food web in a glass box. His goal (which cannot be achieved IMO) was based off of a book written by Walter Adey and Karen Loveland. What Dr. Ron, Eric Borneman, Dr. Toonen, et al failed to tell hobbyists was that Adey had never actually set up an aquarium to test his theories prior to publishing the book. It should also be noted that Adey is a botanist and not a marine biologist.
Many a large scale aquarium or public aquarium has been started using Adey's and Loveland's theoretical work. And every single one of them failed. Since he was backed by the Smithsonian, his first trial with ATS's was the Fort Pierce Smithsonian Aquarium. After a short time, it became necessary to add carbon to remove gelbstoff (yellowing compounds from the algae), then they added mechanical filtration, then they added protein skimmers which went against everything Adey believed in as they were going to remove vital members of the food web.
This initial enthusiasm has been experienced by other public and research large scale aquariums including the Great Barrier Reef Aquarium, The Monaco Aquarium, one in Japan (I THINK Okinawa but I'm not sure so don't quote me on that) and his most recent completed---the Biosphere II. Every single one of these with the exception of Biosphere II has completely changed their filtration methods against the way Adey had them originally set up and went to traditional methods (waterchanges, protein skimmers, activated carbon, and ozone).
Here's some pics of Biosphere II from a couple of years back before the University took it over.
I think it's important that you realize these pictures were taken in
full sunlight and that's how much the water was darkened by all of the gelbstoff released by the algaes. Just about every single coral in that huge tank was killed by algae and even though there are thousands of tangs in there, they cannot keep up with the algae. If you squint real hard, you can see a couple of Hippo Tangs in addition to the yellow tangs.
