Join Date: Jan 2006
Location: Burnaby, BC, Canada
Depending on the size of your tank and the amount of water you change a week, slow release rocks won't help. For instance, I use EI and change 50% of the water in my hi-tech 20 gallon. To buffer KH/GH I added a "tufa" rock. It's working wonderfully, keeping my normally KH/GH = 2/2 water at about 4/5 or so.
In my 125, the rocks are doing nothing, because in that much water, not enough rock is dissolving over that period of time.
The crushed oyster shells will have higher surface area which would allow more rapid dissolution, I think.
Filstar pimp #59 (shhhh on the Eheim)