Is this aquarium cycled?
Diatoms, like all algae plagues, are invariably caused and sustained by excess nutrients in the water column, silica is the most important of these in the case of diatoms. However, removing silica, which can cost serious $$$, is not necessarily the best method for tackling diatom infestations.
IME, is to simply waiting them out. It will go away in time unless you live in an area where the tap water is high in dissolved Silica then you may need to look for another water source