This is real crushed oyster shell calcite, not all other shell aragonite. Or more correctly, `Sheared` to try and keep the low end soft chalky calcium carbonate. That`s why you will see chalky calcite lumps and straight line cuts.
There is no such thing as commercially available crushed oyster shell.  `Chicken feed`  crushed oyster shell is cockle, clam shell(aragonite).  According to trading standards, the wording `crushed oyster shell` is a `blanket descriptive` that includes all bivalve molusc- cockle,clam, scallop, -aragonite.   Look for one sided parallel lines, or`regular nicks` on the inside of shell(which confirms if it is horizonal or vertical) -- vertical cockle,horizontal clam.  The last pic. is X magnified oyster shell calcite matrix. Real Oyster shell is 100% calcite.
This is what falls between 6-9 mm screens and is a nice size for Small mesh nylon pond bags.
 
The in-between  finger pics. is to try and show the soft calcite sandwich.
Get in touch if I can help with any Q.

This product will be dusty(calcium carbonate dust). Dunk rinse in your Spent pond water or bucket swirl until clear to eliminate any chance of sudden `ph change stress` from the dust .  Then,don`t waste the water- feed your roses,lawn.

Bicarb. without doubt has it`s place, but why not try a gentler approach by using both, lessening the height of the see-saw effect. 
Plus the benefits of desirable magnesium carbonate(2-3%), and trace elements found in real oyster shell.

 Please look at feedback on this item. 
I have been supplying `oyster shell for koi` for over 40 yrs.

Price is £2.50 /Kg.