Do you have a link to the page where the rock is at? Because frankly lava rock by itself is completely incapable of lowering or raising pH. It's completely inert. There must be something put into the rock bag your looking at.
Was looking at different alternatives. Thought the volcanite would be good for BB growth.
Dumb idea? Thoughts
The first link looks okay, although I don't really know much about green sand. The rest of the ingredients are basically inert, and shouldn't affect pH.
I have no idea about the second one, the description doesn't really make sense to me, it sounds like the person writing the description doesn't know much about mineralogy, chemistry, or gardening.
In particular, this line troubles me:
Use some around plants and you'll see how irrelevant it is to worry about pH; the pH of lava sand is more than 8.
Although, I suspect the shipping charges will make it more expensive then a bag of flourite or eco complete or whatever from the local pet store.
If there is something like a stone/gravel yard, or a landscape supply around you, I'd check there, some of the ones I've been too have different sized grades of lava rock, and sell it for cheaper then almost anywhere else. (You'll probably need to bring something to put it in). Home Depot/Lowes typically has bags of the 1-3" sized chunks, but you may get lucky and find some smaller sized stuff used for decorating potted plants, etc.
I agree that if you really want this, buy it and test it before adding to your tank. Me personally, I would avoid the shipping and buy at a local landscape or home improvement center. Lava rock is fairly cheap locally and you wouldn't have to pay the shipping charges due to weight.
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I myself is stting up a planted tank and is considering the red lave from HD. I'm a minerologist by training and I know that it is porous, light wight and rich in iron and other nutrients. Farmers in land with weathered lava normally have good crorps. It shold be similar to Florite or Eco comlete in effect, and may be better, but a lot cheaper.
Has any one used it before? Should be very easy to crush to smaller sizes.
I just hammer the lava rock to the size I need and I rinse it. it's better then bo-balls in your filter or mix it in your substrate Im seting up a dirt tank and will be puting small pieces in with the dirt hope this helps.