what exactly is it?

[DarrylR]

Latly the main filter pump at petsmart has been spewing sand like substrate into the tanks, gray, nets can't hold it, and the manager is like it is glass I think he said. And it beneficial and cleans the tank. In the back room where the main filter pump is, like a obelisk pump and inside it has the gray sand like stuff floating around, and I think it was labeled as feeder filter.

My question is, does anyone actually know what it is, and also can you use it as a tank substrate?

 

[wataugachicken]

sounds like diatomaceous earth, in a diatom filter. it is used to polish the water, as it traps anything bigger than the pores in the filter, which are teeny-tiny. it is *sort* of like glass, in the way that both it and glass are made of silica, and it is very very sharp on a microscopic level. definitely don't use it as a substrate. the particles are so fine that they can cause lung diseases in people and animals if inhaled (industrial or pool filter grade DE is more dangerous than organic garden/food grade DE). i don't know how it affects gills but it shouldn't be free in the tanks.

 

[spirals]

fluidized bedding, pretty sure its there to add as an extra filter for finer particles along with biofiltration and its definitely not supposed to be in the tanks. the only time we've had ours in the tank was when we had a massive system shutdown (that was a very sad day).
def. don't want to use that as substrate