Zeolites have been used in this hobby in the USA for over a decade. It is what Nitrate Sponges are made of, e.g. X-Nitrate. As some one has mentioned "kitty litter", which is correct and the same for "water softeners" and "floor dry", all which are made of the same Zeolite_Clinoptilolite_, which is the end mineral of the Heulandite Series, where there has been a replacement of K for NA.. These minerals are often confused with clays, which they are not even related to, other than the fact they are both Al silicates often with water. They belong to or are some times put in the Feldspar Group and are in the same silicate Subclass as Quartz_Tektosilicates_. They are not structured like clay at all. Heulandite/Clinoptilolite is (Na, Ca)2-3 Al3 (Al, Si)2 Si13 O36 : 12H20. Clay mienrals are Phyllosilcates, which form sheets of tetrahedra, each sharing 3O. Tektosilicares are 3-dimensioanl frameworks of tetrahedra, each sharing all 4 O. All clays are also hydoxides and Zeolites are not
The thing about Zeolites is that they can loose all their water without changing their crystallographic structure, where other ions/compounds can replace the water. The general formula is Wm'Zr'O2r'_sH20. Where W is chiefly Na and Ca (K, Ba and Sr lesser). Z is Si + Al, where Si:Al is 1 or greater and s is variable. The ratio Al2O3

CaO + Na2O) is always 1:1 and the (Al + Si):O is always 1:2. There are different composition ranges in different Zeolite minerals but it is narrow. They all have a very open wide meshed structure, where the cavities in the framework contain cations that balance the anions. However, the easy of the exchange is what makes them unique.
Beside the many Natural Zeolites there are 100's of artificial Zeolites. O2 Generators are a good example where 3 different Zeolites are used to remove ambient air gases, while leaving behind O2. A DIY unit can take ambient air and give you a return of 50 % O2 and high-tech units as high as 95 % O2. The word Zeolite is Greek, which means to boil, which is what "appears" to happen when you heat a Zeolite to drive off the water.
Zeolites have almost nil application in seawater due to it's ionic strength. The amount of Nitrate Zeolites can actually adsorb is also nil, if at all. The use of them in this hobby is that they end up acting like LR and bring about Facultative Anaerobic Denitrification, which can be accomplished with any pores media that meets the requirement, like certain GAC's
As far as does the product do what is claimed, that's hard to answer and would like to see some real serious tests. I would also like to see them EXPLAIN how the product really works and just what are the _3 Zeolites_