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The two thresholds

The criteria define two regulatory thresholds, both expressed as Total Ammonia Nitrogen (TAN) — the sum of un-ionized ammonia (NH₃) and ammonium (NH₄⁺), since aquarium test kits measure them together.

• Criterion Continuous Concentration (CCC) — the 30-day average concentration that should not cause unacceptable chronic effects. This is the threshold for healthy long-term keeping.
• Criterion Maximum Concentration (CMC) — the 1-hour average concentration that should not cause unacceptable acute effects (death within hours to days).

Reference-point values (pH 7.0, 20°C)

At the EPA's reference conditions — pH 7.0 and 20°C (68°F), with freshwater mussels and fish early life stages present — the criteria are:

Criterion	Concentration (mg/L TAN)	Averaging period
CCC (chronic)	1.9	30-day rolling average
CMC (acute)	17	1-hour average

At pH 7.0, across temperature

Temperature has a major effect — warmer water shifts more ammonia into the toxic un-ionized form and stresses invertebrates more. At pH 7.0, the criteria across temperature are:

Temperature	CMC (acute, mg/L TAN)	CCC (chronic, mg/L TAN)
0°C (32°F)	24	4.4
30°C (86°F)	7.3	0.99

The full pH × temperature lookup matrix (covering pH 6.5–9.0 and the same temperature range) is published as Tables 5a and 5b in the source document linked below. If your tank is at a pH other than 7.0, use those tables directly rather than interpolating — the relationship is non-linear and the EPA's published values are the authoritative form.

Reading the criteria for aquarium use

Your liquid test kit reports TAN (the sum of NH₃ and NH₄⁺). Compare it to the CCC for your temperature:

• Below CCC — safe for indefinite exposure.
• Between CCC and CMC — stressful; trigger a water change immediately and address the cause.
• Above CMC — emergency. Substantial water change now; the fish are in acute toxicity territory.

Why borrow these numbers at all

The hobby has no equivalent body of testing. The EPA's criteria are the most rigorously derived freshwater ammonia thresholds in print — built from genus-level toxicity tests across native species, including freshwater mussels (notably more sensitive than fish, which is why these numbers got stricter in 2013 when mussel data was added). Until the hobby produces something better, these are the cleanest reference points we've got.

The hobby rule

• Cycled tank — 0 ppm. Anything detectable means your nitrite-oxidizers aren't keeping up. Look for recent overfeeding, a dead fish you missed, substrate disturbance that released gunk, or antibacterial dosing that wiped part of the biofilter.
• During cycling — spike is expected. Nitrite typically appears 1–2 weeks after the ammonia spike, peaks for several days, then drops to 0 over the following 1–2 weeks as the second bacterial colony (Nitrobacter / Nitrospira) catches up.
• Brown blood disease (methemoglobinemia). Nitrite outcompetes oxygen on fish hemoglobin at the gill. Symptoms: tan/brown-tinted gills and blood, fish gasping at the surface even with strong aeration. Acute and fatal — and faster-acting than ammonia toxicity, which is why "cycling kills" usually happen at the nitrite stage, not the ammonia stage.

Salt as a protective trick during cycling

Chloride competes with nitrite at the fish gill. Dosing ~1 ppt salt (roughly 1 teaspoon of aquarium salt per 4 gallons) during the nitrite stage of a fish-in cycle meaningfully reduces uptake and protects the fish through the spike. Caveats — skip this if your stocking includes:

• Caridina shrimp (Crystal Reds, Taiwan Bees, Pintos) — salt-intolerant.
• Scaleless fish — corydoras, loaches, some catfish.
• Salt-sensitive plants — most stem plants and many crypts will degrade.

For sensitive stocking, do a fishless cycle (dose ammonia, no fish in tank) and skip the salt entirely.

EPA drinking-water MCL for nitrite

Contaminant	MCLG	MCL (enforceable)	Codified at
Nitrite (measured as Nitrogen, NO2-N)	1.0 mg/L	1.0 mg/L	40 CFR §141.62

Nitrite's drinking-water MCL is ten times stricter than nitrate's (1.0 vs 10.0 mg/L as N), codified in the same section of the same regulation. The EPA recognizes nitrite as substantially more acutely toxic at lower concentrations — same principle that applies in aquariums, just at a different scale: nitrate hobby caps are 20–40 ppm, nitrite should be 0.

EPA drinking-water Maximum Contaminant Level (MCL)

Contaminant	MCLG	MCL (enforceable)	Codified at
Nitrate (measured as Nitrogen, NO3-N)	10 mg/L	10 mg/L	40 CFR §141.62

A critical unit detail: this 10 mg/L is measured as nitrogen, not as total nitrate ion. To convert to "nitrate as NO₃^-" (which some test kits and overseas standards use), multiply by 4.43. The 10 mg/L NO₃-N equals roughly 44 mg/L NO₃^-.

Why hobby caps come in lower

The drinking-water MCL is set for human consumption — infants are the binding constraint, because nitrate can interfere with infant hemoglobin (methemoglobinemia, "blue baby syndrome"). It was never designed to protect long-term aquatic life. Hobby thresholds of 20–40 ppm exist because chronic exposure stresses fish at concentrations well below the human-health limit, and sensitive shrimp species show reproductive impairment lower still.

Aquarium hobby soft caps

These are not regulatory — they are convergent recommendations from experienced keepers and aquarium veterinary literature. Use them as soft targets, not bright lines.

Stocking	Soft cap (ppm NO3- as N)
Caridina shrimp (Crystal Reds, Taiwan Bees, Pintos)	≤ 10
Neocaridina shrimp (Cherry, Blue Dream, etc.)	≤ 20
Sensitive community fish (rasboras, tetras, corydoras)	≤ 20
Hardy community fish (most livebearers, danios)	≤ 40
Any freshwater inhabitant	< 80 (emergency threshold)

Destroying a contaminated moss ball — choose ONE method

The protocol — pick one:

Method	Procedure
Freeze	Place the moss ball in a sealable plastic bag and freeze for at least 24 hours.
Boil	Place the moss ball in boiling water for at least 1 full minute.
Bleach	Submerge in regular unscented bleach diluted to 1/3 cup per gallon of water, for 10 minutes.
Vinegar	Submerge in undiluted white vinegar for 20 minutes.

Disposal

After destruction, dispose in a sealed plastic bag in the trash. Never down drains, waterways, or compost. The treatment liquids (vinegar, boiling water, bleach solution) may be poured down a household drain — but never a storm drain.

Decontaminating the aquarium itself

If the moss ball was in a stocked tank, the tank and equipment need decon too:

Method	Procedure
Hot water	Flush and coat tank, accessories, and equipment with 120°F water for at least 2 minutes.
Saline soak	1/2 cup salt per gallon; soak substrate, rocks, décor, and filter media for 24 hours.
Bleach contaminated water	1/3 cup bleach per gallon, let sit 10 minutes, then drain to a household drain.

Reporting confirmed contamination

Confirmed contamination should be reported to:

• Your state fish & wildlife agency — they coordinate state-level response and may collect the specimen for confirmation.
• The USGS NAS database (see below) — for the public scientific record.
• USFWS regional office — for federally listed invasive species.

Key URLs

Resource	URL
NAS database homepage	nas.er.usgs.gov
Sighting report form	nas.er.usgs.gov/SightingReport.aspx
Zebra mussel (Dreissena polymorpha) species profile	Zebra mussel fact sheet
Cherry shrimp (Neocaridina davidi) species profile	Cherry shrimp fact sheet

What the sighting form asks for

The form expects a complete record so the database entry is useful to researchers. Fields:

• Species — type (Crustacean / Fish / Mollusk / Plant / etc.), common name, genus, species, number observed
• Date of observation — MM/DD/YYYY (when you saw it, not when you're reporting)
• Location — state, county, latitude/longitude (decimal or DMS), specific location description
• Observer — name, email, telephone, address
• Image — up to 4 photos (.jpg / .png / .heic, 1 MB each max)
• Comments — context like dead-or-alive, where exactly (in the moss ball, in the filter, on substrate)

What makes a useful sighting report

A good sighting report needs five things:

• Location — GPS coordinates or a precise address; "near my house" is not enough for the database to use.
• Date — when you saw it, not when you're reporting.
• Species — best guess at scientific name; the form accepts photos for verification.
• Photo — clear, in-focus, with something for scale.
• Context — was it dead or alive, how many, where exactly (in the moss ball, on the substrate, in the filter)?

EPA Method 350.1 — Ammonia by colorimetry

This is the federally referenced method that most commercial aquarium ammonia test kits implement (in simplified form). It measures Total Ammonia Nitrogen — both NH₃ and NH₄⁺ — using the automated phenate (indophenol blue) reaction: sample reacts with alkaline phenol and hypochlorite, intensified with sodium nitroprusside, absorbance read at 630–660 nm.

APHA Standard Methods — water testing reference

The professional reference for water analysis methodology. Aquarium test kits are usually simplified versions of these methods; the published volume is the authority on what each test actually measures.

Parameter	Section family
Ammonia (NH3 + NH4+ as N)	4500-NH3
Nitrite (NO2- as N)	4500-NO2-
Nitrate (NO3- as N)	4500-NO3-

Each section family contains multiple sub-methods (titrimetric, colorimetric, ion-selective electrode, flow injection). Sub-method letters have shifted between editions; cite by edition when referencing a specific lettered sub-method.