Nitrite, nitrate and ammonia cause problems in every kind of water, but US rules treat them very differently depending on where the water is.
Fish farms are the least regulated. No rule sets nitrite or ammonia limits inside a tank or pond. Larger facilities have discharge permits, mostly built around good management practices. The reason to measure is economic: a nitrogen spike can harm stock within hours, so farmers test because their livelihood depends on it.
Lakes are regulated around the lake rather than in it. Under the Clean Water Act, states set water quality standards and list waters that fail them. Nutrient pollution is one of the most common reasons waters are listed as impaired. But most lakes have no monitoring mandate, and runoff from farmland, the largest source of nitrogen, falls largely outside permit programs. Lake associations, agencies and researchers monitor because they want healthy water, usually on tight budgets.
Treatment plants are the most regulated. Discharge permits commonly set ammonia limits, and many also limit total nitrogen in sensitive watersheds. Results reported to regulators must come from approved laboratory methods. But how operators run their plants day to day, from aeration to chemical dosing, is up to them, and that is where fast, on-site measurement pays off.
Across all three, regulation creates the need, and certified labs provide the record. On-site tools support the daily decisions in between. Precedent also shows the path forward: an enzyme-based nitrate method has already become a US Geological Survey standard method.
Note: this article is general information, not legal advice. Requirements vary by state and permit.