That might be part of it. Aerobic bacteria is nitrifying bacteria - like those found in shallow sand and other oxygenated areas. Thus you would have plenty in your shallow sand bed. Thus increasing the depth would increase the denitrifying bacteria - the anaerobic bacteria. But a DSB in the display isn't the only way. As you know, many people go bare bottom on their DT's. They don't have any anaerobic bacteria on the bottom of the tanks at all. They make up for it with their refugiums most of the time. They use refugium mud, etc which allows a ton of anaerobic bacteria to grow.
So as long as you figure out ways to increase anaerobic bacteria, you'll help things out.
But that's kinda the end of the process. In order to help things even more, you can help prevent the waste byproducts before they start - thus preventing the NH3, N02, and N03 from even being made in the first place. That is by simply increasing waste removal before it can be broken down. Some ways to do this: I see you have a prefilter on your sump. You have to clean that thing very, very regularly to get all the waste out of it. Another way is your skimmer - I see you have one, but skimmers can be quite insufficient for the tanksize they are recommended for. That's why many people use oversized/over-rated skimmers. The more skimmate that you can take off, the more waste you get before it's processed by the bacteria in the tank.
And of course some other things are very simple- decreasing feeding frequency/amounts. Decreasing the parts of the food that aren't eaten - that's why some people strain their food before adding it. etc, etc.
Well, that's just a couple things to give you an idea... Obviously there's not just one answer to fix the problem, but many things together can make a huge difference.
