Well, freshwater is a different beast, in large part because many of the fish are now captive bred and are probably able to tolerate less than pristine natural conditions - if they exist anymore, LOL. Nitrate isn't really as much of a concern for fish as it is some types of inverts all in all, but at very high levels it (along with the poor maintenance that very high levels imply), it is considered to be a possible problem.
Petsmart, FWIW, doesn't make those claims as much as the manufacturers of those products. And frankly many of them are more hype than not. I would be wary of using some of those products if they are made to remove nitrite, nitrate, etc if it is just a bottle of stuff. There are some designed to basically be used in saltwater systems and I would stick with those.
I would also strongly suggest, if possible, to buy from a specialty fish store, or online, instead of getting advice from Petsmart on this. Does petsmart even sell saltwater fish? I know they sell some saltwater products.
But, IMO, the best bet for reducing nitrates is to improve frequency of water changes, cut down on feeding/stocking and add other methods for removal (eg skimmer if needed, refugium, etc). Bottles and resins are not the best choice, IMO.
