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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
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.