That advertisement is so full of BS that it would be funny if people weren't actually spending hard earned money on it. I don't have time to point out all of the ridiculous errors, so let's just take a couple. The ad claims that the device changes the molecular structure of water. Assuming you could change the molecular structure of a stable compound like water using magnets in a plastic tube (which you can't), if you change the molecular structure, it ain't water anymore. The ad claims that the mystery material made from serpentine etc. emits radiation in the infrared, yet there is no power source. To quote engineer Scott "Captain, ya cannot change the laws of physics". No energy in, no energy out (unless it is undergoing radioactive decay). The ad claims to align the polar water molecules. Water is indeed polar, but in liquid water the molecules are in constant motion relative to each other. If they weren't, it would be ice. So even if you could align the water molecules using magnets in a tube, it would last a nanosecond before the molecules jostled each other out of alignment. No known physical property of the water would be changed. I could go on and on.
However, science does not know everything. There may be properties of matter that have not been measured or explained by science. These unknown properties might have an effect on biological systems (like your fish tank). These unknown properties might be improved by magnets in a tube. But I wouldn't bet on it. And I wouldn't buy it from people who are lying about what it does.
By the way, I have a degree in chemistry.
Sorry for the long rant.