zeovit rocks - how often do you replace?

[Delphinus]

Sure seems kind of wasteful to me though. But I imagine you can't really "recharge" them. I just have the hardest time throwing "rocks" into the garbage when you think about all the effort that went into producing them - the mining, the transport, etc. .. I guess in the grand scheme of things it's $15 per litre of rocks so it's not like it's a huge expense but still it just feels odd to me to throw them out. There's only so many times you can toss this stuff under your deck or in your garden..