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1.8 Kg of Real native Oyster Shell sheared not crushed to keep the calcite bottom end chalk sandwich in a 12X13" nylon reusable bag. The shells have been cleaned. All protein hinge and clamp down completely removed. Rinsed many times then heat treated at 200 deg. C. for 60 mins. The bags are double volume so they can be introduced as a 2.5 Lt. lump or 12"X11"X 1.5" Slab. 

I also have bagged1.8Kg smalls listed .  Always willing to combine post if possible or answer Q`s.

Please note. Due to the final heating process a small amount  of pure calcite chalk lumps will float(inside the bag) until they reabsorb with your pond water. You can use as much, or as little as you see fit using the bag provided.

There is no such thing as `crushed oyster shell`. There never has been. Unless you make it yourself. According to Trading Standards, the wording `crushed oyster shell` is a trade name, and it`s use is a collective of all seashells. But, I was told that I was well within my rights to call my products `Real Oyster shell`.

What`s the difference?

Oyster shell is calcite, all other shells are aragonite.

Real Oyster shell calcite(due to it`s softer, more open matrix, magnesium carbonate content), is at least 30% more active/reactive(quicker) to acidic conditions than aragonite.

Oyster shell verses crushed coral/calcified seaweed. Taken from `koivet.com`  Coral/Calcified seaweed is less than a third as active/giving as oyster shell. Plus the benefit of 5-10% Magnesium Carbonate esp. beneficial for sidewall bacteria.  Oyster shell -once it`s gone-it`s gone. Not giving you false confidence of some carbonate cover with end of life residual gunge.