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Onyx Sand Surprise

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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!


    • #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?


      • #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!


        • #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, 16:30.


          • #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!


            • #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%.


              • #8
                I can see that this conversation is going nowhere, since I can’t get replies without dogging you constantly. Let’s just forget it. I have changed out the Seachem product and replaced it with CaribSea’'s “"Peace River"” substrate (which is beautiful). It has none of the dangers associated with Onyx Sand.

                Clearly, Seachem’s marketing was VERY sloppy in ignoring that portion of the planted tank community that injects CO2, and making the broad statement that Onyx Sand has a “slight buffering capacity.” As you know, full well, once the pH drops below 7.0, due to CO2 presence, the Onyx Sand releases calcium and magnesium carbonates galore, making GH and KH control impossible. The statement, by Seachem, that Onyx San has “slight buffering capacity” is completely erroneous in such situations and should not be stated as applicable to all aquariums.

                Most of your products are excellent. I will, simply, have to be VERY cautious about using any of your products for the first time. I will need to check other sources so as not to blindly rely upon your marketing tactics.
                Last edited by Deanna; 01-25-2018, 19:25.


                • #9
                  Hi Deanna,

                  We are sorry for the delay in responding and that you have replaced the Onyx Sand. At a lower pH, the Onyx Sand will certainly have a continuous release of calcium and carbonates, thus increasing your KH and GH in the aquarium. We would typically recommend the Onyx Sand for higher pH environments such as Cichlid Tanks OR as a base layer in a planted tank. If you put it as a thin base layer under another substrate, it will keep the top layer from becoming anoxic by providing those carbonates, all the while keeping the plant roots healthy and preventing root rot.

                  Again we are sorry for your frustrations with Onyx Sand and hope that you will continue to use your other Seachem products. Let us know if you need any assistance in the future.