Diatoms are single celled algae, brownish in color. It starts out as a dusting on the substrate that rapidly turns into a mat that can cover the tank over in just 5 days, provided that they have a surplus on nutrients available.
The cause: excess nutrients, silicate and silicic acid are the ingredients for a diatom bloom. Silicic acid enters the aquarium by using tap water; substrate can contain silicates that leach out over time. RO/DI units can filter silicic acid out, but only for a limited time. Eliminating silicates can control diatoms.