Most play sand is granite sand. Although silica is present in granite, it CANNOT be consumed by diatoms as it is a very tight molecular structure. Just like the glass sides of the aquarium it is not a diatom feeding ground.
Coral sand and rocks are much more likely sources of silica:
Diatoms consume silica
Corals eat diatoms
Corals make their hard structure out of things they consume including the silica from diatoms.
Corals die
Dead corals become rocks and wear down to sand.
Those rocks and sand have molecularly loose silica in them which CAN feed the diatoms
And the circle of diatoms continues.
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