In Biofloc Technology (BFT), the Carbon-to-Nitrogen ratio ($C:N$ ratio) is the single most critical governing factor. Maintaining a high $C:N$ ratio (typically $12:1$ to $15:1$) by external addition of organic carbon sources (such as molasses, wheat flour, or starch) stimulates heterotrophic bacteria to rapidly assimilate toxic inorganic nitrogen ($NH_4^+$ and $NO_2^-$) into microbial single-cell protein (biofloc biomass), purifying the water and serving as supplemental fish feed.

Option analysis:

• Carbon: Correct, supplemental organic carbon drives the entire heterotrophic biofloc process.
• Phosphorus: Essential macronutrient, but not the primary driver of microbial biofloc formation.
• Potash: Required in trace/minor amounts for ion balance.
• Magnesium: Mineral necessary for shrimp osmoregulation, not biofloc synthesis.