OK. I opened a bag of Eco and dumped it in a clean 10g leader. Got a lot of water in it, and the depth was about 1-3/4", or 350 c.i. volume altogether. Cleaned it out, opened a bag of Volcanit and did the same. The Volcanit is dry, made about 1-1/2", or 300 c.i. I would imagine if it had a few quarts of water in it like the Eco it would have settled even lower. I would have done the same with Onyx, but my bag was already partially used. Have attached a pic to give some idea of the color and texture of the 3 substrates.
Volcanit is advertised as black and red. That's even a farther stretch than saying Onyx is black. Marketing. Also, the Volcanit is very inconsistent in particle size, and the larger particles are bigger than any other commercial substrate I've used. It looks sort of like dirt. Unattractive. Won't be buying it again, even if it beats the Eco for growing plants. In the pic of bags it's Volcanit, Onyx, Eco clockwise from the top. The handfuls are Eco, Volcanit, Onyx.