From memory formaldehyde was used in the manufacturing process but not necessary in the product itself. Those 'erasers' are melamine foam and the structure looks almost like steel wool. So that's why it can remove stuff because of it's internal structure. For me I wouldn't use a product like that in such a delicate ecosystem too worried perhaps about what 'might be' during production. One thing that I don't like is that the product actually 'wears down'. Hence you'll now have minute particles of melamine foam in your tank which no doubly some critters will ingest. A few years back there was a story out of China with liquid baby food about someone adding melamine to inflate protein counts and of course went horribly wrong because melamine in any regard should not be consumed. Why the labels say to keep away from kids and pets because it's possible they could ingest the product.
I'm much happier with something like this.
http://www.homedepot.ca/product/razo...-blades/972573
my 2c