Sorry to bring up an old thread but found it as I was searching info for hypersorb.
I've been using purigen for 2 years now and have found the exact same as others have stated. But according to seachem, it doesn't remove nitrates, it removes the organics that lead to nitrates. Hypersorb is less specific than purigen.
Another thing that I saw was the detoxifying of nitrates from amquel+. This will only bind with nitrates for 24 hours. If you do a water test 24 hours later, it will be back to the levels they were before. Which is actually quite a negative thing, because the ammonia/nitrites become unavailable to aerobic bacteria, thus the bacteria die, so when it suddenly becomes available, there's less bacteria to process it. The only time that I use Amquel+ (because I used to love the stuff when I was unaware) now is when I'm doing water changes to reduce the ammonia/nitrites/nitrates and cannot do enough to bring them back into check.
For removing nitrates I've also used Kent Nitrate Sponge and Seachem's de*Nitrate. They encourage the development of aerobic bacteria on the surface so that it will remove the oxygen and allow for anaerobic bacteria inside to process the Nitrogen from the Nitrates. This anaerobic bacteria is the only way that Nitrates can be removed biologically, and it requires a lack of oxygen for them to exist. The best way to remove Nitrate that I have found is by building a coil denitrator, which has even outperformed the Nitrate Sponge or de*Nitrate. Worth googling it if you're really trying to reduce Nitrates. I've found the key is to build multiple coils to allow more water to be de-oxygenated at the same time. Essentially you use coils of tubing placed around a PCP pipe, with it letting water in at the bottom of the PCP pipe. Water travels very slowly (drips per second) so as it travels through the coils, aerobic bacteria line the tubing, removing oxygen. By the time it enters the PCP pipe at the bottom, there's no oxygen and anaerobic bacteria colonize bio balls inside the PCP pipe traveling up and existing at the top back into the tank. Those drips of water will have 0 nitrates which 24/7 can really reduce Nitrates in the tank.