FWIW, I have a 30 gallon that probably has an average depth of 3", which was about 3.5 bags. I bought 4 and don't remember what I did with the extra 1/2 bag. I may have put it in the tank which would actually give me an average depth of 3.5".
In any event, I have a slope in 1 corner that must be almost 6" and I'm not really worried about it.
I think 4" in a section will be fine.
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