Deep sand beds are a well established practice in the salt water hobby.
Basically you have bacteria to eat ammonia, and nitrites, but the bacteria that eats nitrates can only grow in dark nearly anaerobic conditions. I'm paraphrasing from my past research.
A deep sand bed allows for these conditions to exist.
Generally you need 4-6 inch deep sand. You can do this by having a deep substrate, or you can put it in your sump.
I had a DSB set up in the sump of my dads 150 gallon that was stocked with a GT, an Oscar, a Dempsey, a blue crayfish and a dozen or so giant Danios.
He didn't do a water change for a year and a half. All he did was top off evaporation, and his readings never got out of control.
Since you already have a sand substrate, it would be as simple as adding more sand.
There is disagreement on the necessity of it, but some say you should have something to slowly turn throw the substrate to prevent toxic gas pockets.
Personally I just put some trumpet snails in my African tank. The fish will eat them so they hide during the day and come out at night and keep the tank alge free