Ideal aquarium water parameters
The short answer: ammonia and nitrite at zero in any established tank, and everything else depends on what you keep. These are the ranges the analysis judges against for five typical tanks.
| Parameter | Community | Planted with CO₂ | Neocaridina shrimp | Caridina shrimp | Mixed reef (soft, LPS) |
|---|---|---|---|---|---|
| NH₄ Ammonia / ammonium (mg/L) | 0 | 0 | 0 | 0 | 0 |
| NO₂ Nitrite (mg/L) | ≤ 0.05 | ≤ 0.05 | ≤ 0.025 | ≤ 0.025 | ≤ 0.1 |
| NO₃ Nitrate (mg/L) | ≤ 25 | 10–40 | ≤ 25 | ≤ 20 | 2–15 |
| PO₄ Phosphate (mg/L) | ≤ 1 | 0.5–3 | ≤ 1 | ≤ 1 | 0.02–0.1 |
| SiO₂ Silicate (mg/L) | ≤ 2 | ≤ 2 | ≤ 2 | ≤ 2 | ≤ 0.5 |
| pH | 6–8 | 5.8–8 | 6.5–8 | 5.8–6.8 | 7.9–8.4 |
| KH Carbonate hardness / alkalinity (dKH) | 3–12 | 2–10 | 2–8 | 0–2 | 7–11 |
| GH General hardness (dGH) | 4–16 | 4–16 | 6–12 | 4–6 | – |
| Temp Temperature (°C) | 22–28 | 22–28 | 18–27 | 20–24 | 24–27 |
| SG Salinity (sg) | – | – | – | – | 1.023–1.026 |
| Ca Calcium (mg/L) | – | – | – | – | 380–460 |
| Mg Magnesium (mg/L) | – | – | – | – | 1,250–1,450 |
Ammonia is total ammonia as a test kit reads it; the analysis actually judges the toxic free ammonia worked out from it, so a small reading can still pass at low pH. Values in mg/L unless stated.
How to read an aquarium water test
Not every number on a test kit carries the same weight. Ammonia and nitrite can kill fish within days and always come first. Nitrate, phosphate and silicate are slower: they decide whether algae or plants win. pH, KH and GH describe the kind of water you keep, and matter mostly when they do not match your livestock or when they swing. The analysis follows the same order, and the overall verdict is set by the worst reading, never by an average.
Ammonia: why pH changes everything
Test kits read total ammonia, the sum of ammonium (NH₄⁺), which is mostly harmless, and free ammonia (NH₃), which burns gills. How the total splits depends on pH and temperature. That is why the analysis converts your reading into free ammonia before judging it, and why it asks for pH:
| pH at 25 °C | Toxic share (NH₃) | 1 mg/L total becomes |
|---|---|---|
| 6.5 | 0.2% | 0.002 mg/L |
| 7.0 | 0.6% | 0.005 mg/L |
| 7.5 | 1.8% | 0.017 mg/L |
| 8.0 | 5.4% | 0.051 mg/L |
| 8.5 | 15% | 0.144 mg/L |
Above 0.02 mg/L of free ammonia fish are stressed, above 0.05 they are harmed, and shrimp react at half those levels. In an established tank any measurable ammonia is still a warning sign: something is decaying, and a water change that raises the pH turns the harmless part into the toxic one.
Nitrite: the danger of new tanks
Nitrite stops fish blood from carrying oxygen. In a mature freshwater tank it should read zero; from 0.2 mg/L it is a problem and above 1 mg/L an emergency. In a tank that is still cycling it is expected, and the analysis says so instead of raising the alarm. Marine fish tolerate it far better, because the chloride in sea water blocks its uptake.
Nitrate: it depends on the tank
Nitrate is the end product of the cycle. Fish tolerate a fair amount, but it feeds algae, and in a planted tank it is food: too little and plants stall. That is why the target moves with the tank:
| Tank | Nitrate target (mg/L) |
|---|---|
| Fish only, no plants | < 20 |
| Planted, no CO₂ | 5 – 30 |
| Planted with CO₂ | 10 – 40 |
| Caridina shrimp | < 20 |
| Mixed reef | 2 – 15 |
| SPS reef | 1 – 10 |
Phosphate and silicate: algae fuel
Neither is toxic at aquarium levels, but both feed algae. Without plants, keep phosphate under 0.5-1 mg/L. In a planted tank it is a nutrient like nitrate, and the analysis also checks the balance between the two: when one runs out first, the typical algae of that imbalance follow. Silicate feeds diatoms, the brown dust on glass in new tanks, and only ever earns a warning. In a reef the bar is much lower: phosphate between 0.02 and 0.1 mg/L.
pH, KH and GH
These describe the water rather than its pollution. KH, the carbonate hardness, holds pH steady: under 2 dKH pH can crash overnight. GH, the general hardness, carries the calcium and magnesium shrimp and snails need for shells. The right values depend on who you keep: Caridina shrimp want KH 0-2 and pH under 7, Rift Lake cichlids want the opposite. Tell the analysis who lives in your tank and it judges against their range, and warns you if two of them want incompatible water.
Marine tanks: salinity, alkalinity, calcium and magnesium
Switch to Marine and the form asks for salinity in the basic step and the big three of reef keeping in the advanced one: alkalinity (7-11 dKH), calcium (380-460 mg/L) and magnesium (1250-1450 mg/L). A fish-only tank is judged more leniently on all three and on nutrients; an SPS reef is held to the tightest nitrate and phosphate targets.
Share your results
Every analysis gets its own link, which contains the readings themselves: send it to a friend, your local shop or a forum, and they see the same analysis in their own language. Nothing is stored. With a free account you can also save the readings to your tank to follow them over time, and download a summary image for Instagram or Facebook.
How the analysis works, and its sources
Each reading is compared with ranges for your kind of tank: a base range for the water type and the planting, plus the range of each inhabitant you tick, and the strictest one wins. For pH, hardness and temperature the inhabitants' own ranges replace the generic one, because a Caridina tank at KH 0 is exactly right. The overall verdict is the worst single reading, never an average. Two values are calculated rather than read: free ammonia, from total ammonia, pH and temperature, and dissolved CO₂, from pH and KH.
The ranges are the consensus of hobby practice, cross-checked against the scientific literature where it exists:
- Emerson K., Russo R.C., Lund R.E., Thurston R.V. (1975). Aqueous ammonia equilibrium calculations: effect of pH and temperature. Journal of the Fisheries Research Board of Canada, 32(12), 2379–2383.
- U.S. Environmental Protection Agency (2013). Aquatic Life Ambient Water Quality Criteria for Ammonia – Freshwater. EPA 822-R-13-001.
- Lewis W.M., Morris D.P. (1986). Toxicity of nitrite to fish: a review. Transactions of the American Fisheries Society, 115(2), 183–195.
- Holmes-Farley R. (2004). Reef aquarium water parameters. Reefkeeping Magazine.
- The pH/KH/CO₂ relationship popularised by The Krib, the same one used by our CO₂ calculator.
It is guidance, not a diagnosis: hobby test kits can be off by a colour step, so retest anything surprising before acting.