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When they can use charcoal? Doesn't charcoal have way more surface area than ceramic rings, which is the point of using them? Charcoal is extremely porous, which is why it's used in filtration. I don't get why people buy those tiny 15 dollar bags of ceramic rings when you can get a huge 5...

I know this is simple, but my mind is in a rut. When estimating the mean of a population, how large a sample is required in order that the 95% error margin be 1/4 of the population standard deviation I'm not what work to show