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Re: Flourite and Onyx

Gary Lange had some questions regarding Seachem's Flouorite and Onyx.
"I didn't see any recommendations for Onyx sand amounts."

At the beginning of summer, I set up a tank using Seachem's Onyx Sand.
According to the package, 1 bag of Onyx Sand (7 kg) is sufficient to cover
217 sq in @ 2" =  434 cubic inches. The substrate bed in my "cube tank"
measures 18 x 19 x 3 = 1083 cubic inches of Onyx. It took 4 full bags of
Onyx Sand to give me a 3" substrate layer, so err on the generous side when
you buy.

When you wash the stuff, be careful - I started washing it using a very fine
mesh sieve (almost a chinois) and found that a large portion of the first
few handfuls went right down the drain. I had to resort to washing it in a
bucket instead. It has a very good gradient mix of individual particle sizes
and thankfully, once in place, it has enough density to stay in place (i.e.
its not like Shultz Clay Soil Conditioner which moves around a lot). Since
it is relatively "expensive", you don't want to flush any of it down the

I hope that you realize that it isn't black - its a dark neutral gray. After
years of using warmer toned substrates (like Flouorite), it was a bit of a
shock at first but I find that the neutrality of the Onyx creates a very
effective foil and backdrop for the lush and vivid greens and reds of the

"I think I remember a higher initial KH with the Onyx sand."

Onyx will affect both the Total Hardness and the Alkalinity of your
aquarium's water. Here are the latest (like, from 20 minutes ago) numbers I
come up with here. The tapwater had been drawn yesterday and is in a holding
tank (I usually let water of water changes sit for a day to get rid of any
dissolved gasses and let it equilibrate with the atmosphere). The tests were
run using the HACH
Total Alkalinity Test Kit and the HACH Hardness, Total & Calcium Test Kit.

Toronto Tap Water -
- Total Hardness 8*20 = 160 mg/L CaCO3 = 9 dGH
- Calcium Hardness 5*20 = 100 mg/L CaCO3
- Magnesium Hardness 3 *20 = 60 mg/L CaCO3
- Total Alkalinity 17*5 = 85 mg/L CaCO3 = 4.8 dKH
Aquarium -
- Total Hardness 13*20 = 260 mg/L CaCO3  = 14.6 dGH
- Calcium Hardness 9*20 = 180 mg/L CaCO3
- Magnesium Hardness 4*20 = 80 mg/L CaCO3
- Total Alkalinity 26*5 = 130 mg/L CaCO3 = 7.3 dKH

My water change schedule on this tank (it holds about 25 gallons of water)
is 15% every 2 days. I _assume_ that I would see more of an increase to
Hardness and Alkalinity if I didn't do partial water changes as regularly as
I do, but I don't know how far either parameter would go if you are lazy and
don't change water regularly.

Filtration is via a small external Eheim Power Filter (like a Hagen
Aquaclear, except it only has a large sponge for media. I don't know if this
model is available in the US.) The tank is CO2 injected via a powerhead and
is sitting next to a north facing window wall (lots of natural light, but no
sunlight except at sunset for a few minutes at this time of year). It is
illuminated by a 175W suspended Metal Halide Pendant (take THAT, you
hi-light wannabe's.... 6.7 W/gal!!!!!!). The tank has only been running for
about 2 months. As can be expected, given the horrendous amount of light
this system receives, the first few weeks saw some algae, but by allowing a
lotus to send up floating leaves (I only allow 4 or 5 floating leaves at any
one time) and by using Salvinia (which has to be skimmed off weekly) I am
able to moderate the light reaching the tank interior and I no longer have
any problem with algae infesting anything..

Plant growth has so far been nothing short of phenomenal. Initial planting
consisted mainly of stem plants like Hygrophila polysperma (the plain green
and the Rosanervig cultivar), Limnophilia sessiliflora, and Bacopa
caroliniana, with a few strong growing rosette type plants thrown in as
well - Echinodorus Ozelot, E. quadricosta, Nymphea lotus, Nuphar japonica,
and in order to tempt fate, I also planted a few Crypts to see how badly
they would "melt". As expected, they did, but they have come back strong and
are doing very well (Cryptocoryne x willisii "lucens", C. parva, C.
pontederiifolia). This weekend I plan on starting to remove most of the stem
plants gradually, and replace them with more rosette type plants (the stem
plants grow too fast - and under this light, the internode length on both
the H. polysperma and L. sessiliflora is elongated and not all that
attractive. The internode length problem might also have been caused by a
month of very high temperatures during this past summer's heat wave).

"Any cory people want to comment, is this stuff too sharp on their

I don't have any catfish in the tank right now, but I don't think you need
worry - the Onyx is not "hard edged". When I washed it, I used my bare hands
to swirl it around in the bucket. It didn't hurt my hands - but had I done
the same thing to wash the crushed granite which I used in my big tank, my
fingers would have bled.

I have nothing "negative" to say about the Onyx - although I will admit to
being skeptical about it at first. I have some growing Blue Diamond Discus
and Dwarf Rams in this tank and they don't seem to mind the effects the Onyx
is having on the water (the regular water changes might have something to do
with that...). As a matter of fact, the Rams feel so at home that they
decided to spawn last week. If you've never seen a 1.5" Dwarf Ram hold a
pack of  4 hungry 2" - 3" diameter Discus at bay, its a sight to behold....

Substrate of the quality of Flouorite and Onyx might seem expensive at
first. But it is a one time hit - it isn't going to have to be changed and
can be re-used over and over. If I end up breaking this particular tank down
at some point in time, I can always re-use the Onyx in another tank.

James Purchase