No experience with oyster shells, but the speed will depend on how well it is crushed and whether it is placed in good flow or not.
Also, the snail shells will erode and they'll die if you can't get the pH above 7 (in addition to upping GH).
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Also, the snail shells will erode and they'll die if you can't get the pH above 7 (in addition to upping GH).
Sent from my Nexus 6 using Tapatalk