Addressing the stone elephant in the room:
It seems to me that this stone is a calcium carbonate stone like the rock we usually use for salt tanks. However there is a chance of leaching trace elements. It sounds like these elements in the rocks aren't toxic and thus their use in freshwater. The worry with these rocks is they also contain a high amount of quartz (SiO2) which means it will leach silicates but then sand is mostly quartz and is far more porous than this rock. Obviously the worry with silicates is diatoms but I already dose copepods heavily in my tanks so hopefully that will control that. Saying that I don't know what else will leach that isn't written about.
As for Everyday Aquarist my knowledge as an environmental engineer makes me wonder if lava rocks leach iron, as iron is often a limiting element in the oceans. It is known to be an element that causes Algea blooms in the ocean and thus why the dust off the Sahara desert, which is high in iron, is such an important environmental function. Allowing Algea growth which in turn absorbs carbon from the air and oceans. If his lava rocks which often contain a hodgepodge of different elements do contain soluble iron. Obviously just a blind hypothesis with absolutely no proof to back it up.
It seems to me that this stone is a calcium carbonate stone like the rock we usually use for salt tanks. However there is a chance of leaching trace elements. It sounds like these elements in the rocks aren't toxic and thus their use in freshwater. The worry with these rocks is they also contain a high amount of quartz (SiO2) which means it will leach silicates but then sand is mostly quartz and is far more porous than this rock. Obviously the worry with silicates is diatoms but I already dose copepods heavily in my tanks so hopefully that will control that. Saying that I don't know what else will leach that isn't written about.
This would potentially explain your diatom problem. (but again why don't we have that problem with sand??)I would be very cautious using the mountain stone. The last time I used freshwater stone (lava, slate, seiryu, etc.) in a marine system, it never left the diatom phase. @EverydayAquarist also has issues with lava rock in his display:
As for Everyday Aquarist my knowledge as an environmental engineer makes me wonder if lava rocks leach iron, as iron is often a limiting element in the oceans. It is known to be an element that causes Algea blooms in the ocean and thus why the dust off the Sahara desert, which is high in iron, is such an important environmental function. Allowing Algea growth which in turn absorbs carbon from the air and oceans. If his lava rocks which often contain a hodgepodge of different elements do contain soluble iron. Obviously just a blind hypothesis with absolutely no proof to back it up.