The first 2 sentences on Seachem's site describing
Matrix read:
"Matrixâ„¢ is a high porosity biomedia that provides efficient biofiltration for the removal of nitrogenous waste. Matrixâ„¢ is a porous inorganic solid about 10 mm in diameter."
Inorganic does not equal stone, stating the obvious, and Matrix ranges in size so the 10mm reference is not accurate either.
The
link posted above would seem to allude to it being a natural, inert stone.
Information regarding recommended flow rates is also no longer included which might indicate a change in the media or simply that the marketing guys have had their hands on it :uhoh3:
It's quite possible that what was introduced as Matrix was replaced by a naturally sourced stone. One of the most popular pond media from Japan was originally a manufactured product and is now a natural stone, yet the product name remains the same.
Which will do a better job of denitrification depends on all the variables in that equation. The availability of carbon and nitrate in the water and the velocity of the water over the media. Siporax is much more capable given the right set up.
Availability of the media is also an obvious limitation here. Siporax has been on the market nearly 20 years. While you could walk into Walmart and buy it a decade ago it's no longer sold in the U.S.. You can source it from ebay through small retailers but that's at a premium price. Matrix is readily available.