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Cito l'intervento di Habib...:
"IMO any sandbed is a sink for nutrients.
If there are not much nutrients (detritus or other sort of nutrients) then they cannot get in the sandbed. But if there are they will.
For example here in Holland but also many other countries we clean out the sandbed (deep beds are very scarce on the Europe except for U.K.).
When we clean it out, say every month, a lot of detritus and strongly colored water is removed.
If a nutrient poor system is what one wants then one should seriously consider a system without major non-biological sinks for nutrients.
Of course one has to remove detritus from the bare bottom and the water column and also dissolved nutrients from the water column.
IMO a sandbed is a reservoir for nutrients and acts like a buffer for such nutrients.
A buffer with very complex properties.
If the sink has become saturated the water can start to get suddenly polluted and can cause serious problems.
Compare it to a tank which really needs a skimmer to keep it low in nutrients and the skimmer stops working but is not noticed. In a matter of days or weeks the system might be heavily polluted.
Assume that there are also pumps to aerate the tank so that aeration is not the problem."
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