Originally Posted by RainSong
Not sure if I agree ... I have 2 tanks. The one with the Eco-Complete substrate is covered in diatoms. The one with the sand substrate has no diatoms. WC for both tanks comes from the same source (tap).
Judging from your locale, your tap water is very high in silica. Any water source filtering through the cascades, columbia basin, or palouse is going though rock that used to be ocean floor before it came up as magma, ash, tephra, or lava.
Freshwater diatoms are always present in tap water. They're enriched for by conditions that include, but are not limited to, low light, pH 6.8, low kH, low Ca, low Mg, high silica, and high concentrations of normally environmentally limiting nutrients found in high concentrations in fish foods, such as phosphate and iron in an organically available form.
Clearly, one of your tanks has those conditions and the other doesn't.