aragamax sand?

Aragamax is aragonite sand, a carbonate sand that will buffer and likely raise your pH. It doesn't have any denitrifying properties by itself but it's small size makes it an ideal home for both aerobic and anaerobic bacterias involved in the nitrogen cycle. The anaerobic bacteria, located in the lower section of deep substrates where the oxygen levels are much lower due to the compact sand, is what reduced nitrates.

In a freshwater setup, we generally don't want an anaerobic area in the substrate due to the toxic gas that the anaerobic bacteria release. The many organisms living in the substrate of a saltwater tank alleviates that problem for saltwater tanks by burrowing through the sand and releasing the gas before it can become a lethal pocket. The same is possible in a freshwater setup, but most do not consider it worth the effort or risk since water changes in a freshwater setup is much cheaper than saltwater.