The color of Onyx sand is akin to old, washed out asphalt. Or maybe cigarette ashes. There are hints of lavender in it as well. Naturally lighting will affect the varying hues; but still, to call something "Onyx" Sand and have it be hardly a medium gray color is really off the hook. I felt quite mislead.
The texture is interesting - almost a little clumpy yet it doesn't compact like play sand. It is great for setting stem plants - it just quickly and easily holds onto whatever you shove into it. You can plant things one handed!
It never, ever rinsed clean. It just kept leaving this milky foam that was almost like wet concrete. By the time I simply gave up and put it in the tank, it was still not rinsing the slightest bit clean. And now every time I vacuum or disturb it in any way, it kicks up clouds of dust that take a little while to settle back down.
It is very uneven in grain size - some particles are practically dust while others are almost the size of medium grade gravel.
There are many sharp edges - every cory I attempted to keep in my tank while using the stuff ended up with no barbels and died. I am not the only one to report this experience.
It promotes hard water and raises pH.
Plant growth seemed nice compared to other substrates.
That is about as impartial as I can be.
Truth is I dislike the stuff very much. The color is so unnatural and the texture is sharp and rough. It made my tank look far from an Amazonian system should, plus it killed all of the corys I put in. At the time when I purchased it, SeaChem hadn't mentioned anything about buffers - and I had even tested it with hydrochloric acid before using it but didn't see any signs of it being calcerous.
Yet my tank never seemed to achieve blackwater status. For months I was puzzled as to why I had extremely hard, alkaline water - then one day I read on Seachem's website that suddenly Onyx Sand has "slight" buffering capabilities. Slight is an understatement and a half. If you can tell me how my source water is dead neutral and I am running heavy peat filtration yet I come up with extremely hard water with a pH of over 7.8 with a "slight" buffer I will give you some serious props. In my estimation, Onyx Sand is almost as strong a buffer as any aragonite or crushed coral.
I'm not meaning to rant here - you might find the stuff to be great. But as for me, I have long since removed all of it from my tank and switched over to a mixture of Tahitian Moon Sand, Quikrete Medium Sand, and Soilmaster Select. Not only does this look infinitely more attractive and natural, but my new corys are happily digging around and my water conditions are exactly as I want them.
Now I have like a hundred pounds of this Onyx Sand and no idea what to do with it. Too bad I can't recoup all the money I wasted on this crap.
But hey...like I said, you might love it. And I'm half considering its use in an upcoming cenote biotope that I have been pondering for several months now...it might actually fit the bill for this, considering I need something with buffering abilities and a sandy gray color...
