carbon
The bio-theory that you're speaking about Rob is due to the fact that people often use carbon in a canister filter type arrangment. Detritus gets caught in the carbon, then breaks down there potentially causing an increase in nitrates.
I suspect that if you just dropped a carbon bag in your sump or main tank that you would not be contributing to increases in your nitrates at all.
I use carbon for about a week or two each month, or until the carbon bag starts to get clogged in my canister filter (whichever comes first)
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