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Ok, I get this. What I don't get is what makes a 1" sand bed different from a 4" sand bed? More surface area. So if adding GFO removes PO4 faster from rocks because of the lack of surface area compared to sand, wouldn't it just take longer? At the same time, limiting the redfield ratio, limiting the production of beneficial bacteria, limiting nutrient reduction and fueling cyano?PO4 literally needs CaCO3 surface area to bond to. Sand has more surface area than a similar volume of rock, so....
Check out what's labelled "Table 2.01" in this blog:
http://blog.brightagrotech.com/biological-surface-area-in-aquaponics/
It's a table that compares the surface area of some common substrates, including medium sand and 4" rocks.
According to that table:
- sand has about 270 square feet of surface area per cubic foot.
- rock has about 12 square feet of surface area per cubic foot.