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  • Onyx Sand Surprise

    I have a 29 gal tank and just added a bag of Onyx Sand to my long-established Flourite. I was expecting an increase in KH (which I got) since your website states: has a “slight buffering capacity.” However, my GH more than doubled from 4 dGH to 9 dGH! This is not buffering, this is hardness. Can you please advise as to what part of the hardness is being increased? Is it the Mg or Ca, or both and by how much? If it is all Ca, then I’ll need to watch my Mg to make sure it is high enough. Also, how long will the GH be affected by the Onyx Sand?

  • #2
    Thanks for the post Deanna! The Onyx sand actually contains higher levels of Calcium and Magnesium than other substrates. This is where the slight buffering capacity comes from - increased Magnesium will stop Calcium and Carbonates from binding and falling out of solution, while the Calcium Carbonate present buffers the water upward. Usually this increase in GH is temporary, and with regular water changes you should start seeing the GH go back down after about a month.

    I hope this helps!

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    • #3
      Ok, thanks. You really should advise potential customers, in your product information page, to expect a sudden surge in their GH when first using this product.

      Can you tell me what the Ca:Mg ratio, generated by the Onyx Sand, is during this 'break-in' period?

      Can this GH problem be resolved faster by placing a bag of the Onyx Sand in a bucket and changing the water, perhaps daily?

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      • #4
        Hi there Deanna, thanks for the follow-up! The Onyx Sand will have a calcium content of 197500 mg/kg and a magnesium content of 115400 mg/kg. As far as the amount of calcium and magnesium you will see in your tank, that will differ widely so I cannot provide a set ratio. Pre-soaking the substrate should help alleviate the potential surge in GH, and I'll definitely make a suggestion to put that somewhere on our website! :)

        I hope this helps!

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        • #5
          Well, there has been none of the indicated reduction in GH. You stated one-month and I am now past two months with no change. GH rises, within three days from 4 dGH (two 60% weekly water changes using RODI water) to 10 dGH and beyond. Contrary to your statement, this is NOT a “slight buffering capacity.” This is a dramatic doubling or tripling of GH. KH also triples, contrary to Seachem’s use of the word; “"slight", but I am not as concerned about KH.

          Can you now, please, give me an ACCURATE assessment as to whether or not the Onyx Sand will EVER stop turning the tank into a hard water tank? If you don’t know, please consult with someone that does. I want to make a decision as to whether or not I need to tear down the tank to remove this liquid rock you call Onyx Sand.

          Incidentally, I notice that Seachem still has not put the warning on the product page about the significant effect that this product has on water parameters. I don’t blame you, no one would buy the product if they knew the danger, but it is very unlike Seachem, at least the old Seachem, to sell your products in this way. I have to say that my confidence in Seachem’s marketing veracity and technical expertise has been severely shaken by this incident.
          Last edited by Deanna; 12-16-2017, 17:30.

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          • #6
            Thanks for the follow-up, Deanna and sorry about the delay in getting back to you! I have a few further questions if you don't mind. At what pH do you keep your tank? How frequently do you perform water changes? Is this a planted tank - if so how heavily planted and what species?

            I look forward to your response!

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            • #7
              It's a hi-tech heavily planted 29-gal tank 60% carpeted with DHG and dwarf sag. The rest are stems: cabomba, bacopa, rotala, anacharis, alternanthera reineckii, stream bogmoss and limnophila hippuroides. Looking down from the top, you would be hard-pressed to see much of the substrate. All are very healthy. Because of the CO2 injection, pH drops from ~7.5 at night to ~6.4 during the day and, unquestionably, the acid is dissolving the carbonates during the day. Weekly water changes are 60%.

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