If it is not a floating nuisance, I prefer the pumice over other forms of lava gravel for one simple reason. It has less of other elements, like iron, manganese, etc. The pale gray stuff I just picked up is mostly silicon dioxide, I think, with very few impurities. [Based on the density in the tubs, I am betting that not much of it floats.] It will not change the water chemistry with a lot of dissolving minerals.
My tap water has adequate iron (over 300 ppb) and other nutrients can be dosed more accurately with the pumice. I see no other reason for not using the lava. Red lava is very popular among many emersed plant growers, I think, like orchid cultivators. It and black lava certainly provide a better substrate color than the stark gray-white of this pumice.
Wright
01 760 872-3995
805 Valley West Circle
Bishop, CA 93514 USA
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