I do 20 gal biweekly which is about 10% of my total water capicity, (more like 15 - 20% if all the rock is put in the equation). I used to do 20 gal weekly on 160 gal but I started to think that was a waste of my time and switched to biweekly after I plumbed in my refugium. The system certainly hasn't suffered from it.
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Greg
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