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My experience with Onyx.

I recently replaced the substrate in my 65 gallon tank (CF light, CO2, TMG) 
with Onyx Sand and just pulled it out a few days ago. For those unaware, 
there is Onyx Gravel and Onyx Sand. Onyx Gravel and Onyx Sand have a wide 
range of particle sizes in them. Onyx Gravel has pieces big as stones to 
perhaps Fluorite sized pieces. Onyx sand has smaller Flourite size pieces to 
dust size particles. I personally think Onyx Gravel is too big to make a good 
planting medium in an aquarium, and so chose the Onyx Sand as it appeared to 
be more suitable for plant roots with it's overall grain size. My experience 
with it was less than satisfactory in a planted tank situation, not because 
of the _quality_ of the product, but because of two characteristics of the 
medium:

1.  The Onyx Sand compacts so tightly that little, if any, detritus seems to 
be able to penetrate the gravel bed and accumulates on the surface. I have 
never had this happen with Fluorite, which has a much more uniform particle 
size to it and enough open space so junk can get down into the bed. Perhaps 
this is an aesthetic thing only, but I wonder wether the gravel bed stays 
mostly anaerobic as a result of the compaction and how many nutrients are 
getting into the gravel bed via entrance betwen substrate particles? 

2. Dust from the Onyx stays on the surface of the substrate, and cannot sink 
below the top particles because of the compaction issue, as does Fluorite 
dust. You will never eliminate all the dust from Onyx or Fluorite. I rinsed 
the Onyx until the water ran clear, but there will always be some dust. The 
tank was clear within an hour of being filled. The problem is from fish that 
root thru the substrate when they feed. I have three corys and a small botia, 
and whenever they would feed, they would kick up dust that coated all the 
plant leaves in the vicinity of their activity. Made the plants look dirty 
all the time.

The main issue I had with the Onyx was one of aesthetics. The Onyx is a 
beautiful color, and although it does buffer the water some, that was not a 
problem. The color was the main reason I tried it.

I wouldn't have any issues with it if the particle size were equal to 
Fluorite, but for me it is a poor choice as a complete substrate for plants. 
Perhaps as part of a mixture with larger grained particles, but not by 
itself. It just makes the plants look dirty and too much junk collects on the 
surface as it doesn't allow the dust to and detritus to sink below the top of 
the substrate. I love regular Fluorite and use it as the only substrate in my 
125 and 65 gallon tanks. It would be nice if Onyx could be produced with a 
more standard grain size similar to Fluorite.  I'm sure it would work great 
in African Cichlid or reef/salt setups. Just my personal experience. I use 
many other SeaChem products in my plant tanks.
Dave