John, I think you are referring to the stuff made for tanks that some people may call "nori" - real nori is actually a naturally growing seaweed. Here is an extract from an encyclopedia that I found.
"Nori (Japanese: 海苔), known in Chinese as hǎitái (海苔) and in Korean as kim or gim (김), is the Japanese name for various edible seaweed species of the red alga Porphyra including most notably P. yezoensis and P. tenera. The term nori is also commonly used to refer to the food products created from these so-called "sea vegetables". Finished products are made by a shredding and rack-drying process that resembles papermaking. Japan, Korea, and China are the current major producers of nori, with total production valued at up to US $2 billion per year.
Production and processing of nori by current methods is a highly advanced form of agriculture. The biology of Porphyra, although complicated, is well understood and this knowledge is used to control virtually every step of the production process. Farming takes place in the sea where the Porphyra plants grow attached to nets suspended at the sea surface and where the farmers operate from boats. The plants grow rapidly, requiring about 45 days from "seeding" until the first harvest. Multiple harvests can be taken from a single seeding, typically at about 10 day intervals. Harvesting is accomplished using mechanical harvesters of a variety of configurations. Processing of raw product is mostly accomplished by highly automated machines that accurately duplicate traditional manual processing steps, but with much improved efficiency and consistency. The final product is a paper thin, dark, black, dried sheet of approximately 18 X 20 cm and 3 grams in weight.
Nori is commonly used as a wrap for sushi makizushi and rice balls onigiri. It is also a common garnish or flavoring in noodle preparations and soups."
