Not diatoms, IMHO. I think that's the stuff that we referred to back in the day as "mulm". It was suggested that a healthy layer of mulm on the bottom of a tank was a prerequisite for breeding many of the more sensitive species of fish, especially those that had very small fry, since it provided a rich source of microscopic food, both living and not-so-living. It was described as a fine layer of organic debris from decaying plant matter, uneaten food and "other sources". Those other sources...were the buttholes of fish...but details like that were rarely mentioned...
But the stuff works as advertised. When I bring fry in from outdoor stock tanks, if they are still down in the 1/4 to 3/8 range or smaller, I always scoop up a couple netfuls of this schmutz (another scientific term...) off the bottom of the stock tank and dump it into the indoor aquarium with them. Most species of fry can be seen actively picking at and searching through this stuff for food.
I'm still surprised that the outfits selling bacteria-in-a-bottle aren't also offering this stuff for sale; probably just waiting until they come up with a catchy name for it. Bottled Mulm sounds so pedestrian...I dunno, maybe "S**t Stew"?