01
Ideal aquarium water parameters at a glance
The table below covers what to test in almost every freshwater tank. The first three rows are about safety: any ammonia or nitrite in an established tank means something is wrong. The rest are about matching your water to your fish. If your tank is new, read how to cycle a fish tank first, because these targets assume a fully cycled filter.
| Parameter | Ideal | Acceptable | Act now if |
|---|---|---|---|
| Ammonia (NH3/NH4+) | 0 ppm | Trace only in a cycling tank | Above 0.25 ppm with fish in the tank |
| Nitrite (NO2-) | 0 ppm | Trace only in a cycling tank | Above 0.25 ppm with fish in the tank |
| Nitrate (NO3-) | Below 20 ppm | Up to 40 ppm | Above 40–50 ppm, or 20 ppm for sensitive species |
| pH | 6.5–7.5 | 6.0–8.0 if stable | Swings of more than 0.3–0.5 in a day |
| GH (general hardness) | 4–12 °dH (70–215 ppm) | Depends on species | Far outside your species' range |
| KH (carbonate hardness) | 3–8 °dH (55–140 ppm) | 2–12 °dH | Below 2 °dH (pH crash risk) |
| Temperature | 76–80 °F (24–27 °C) | 72–82 °F (22–28 °C) for most tropicals | Daily swings above 2 °F (1 °C) |
| Chlorine / chloramine | 0 | 0 | Any: dose conditioner immediately |
02
Ideal water parameters by fish group
Most captive-bred fish adapt to a wide range of water, so you rarely need to chase exact numbers. Still, grouping fish that like similar water makes everything easier. Use this table to plan a community or to check whether your tap water suits the fish you want. Temperature details are in our fish tank temperature guide.
| Group | Temperature | pH | GH (°dH) | KH (°dH) |
|---|---|---|---|---|
| Tropical community (tetras, rasboras, danios) | 75–80 °F (24–27 °C) | 6.5–7.5 | 4–12 | 3–8 |
| Soft-water South Americans (cardinal tetra, apistogramma, ram) | 76–84 °F (24–29 °C) | 5.5–7.0 | 1–8 | 0–4 |
| Discus | 82–86 °F (28–30 °C) | 6.0–7.0 | 1–8 | 0–4 |
| Livebearers (guppy, platy, molly, swordtail) | 72–80 °F (22–27 °C) | 7.0–8.0 | 8–18 | 4–12 |
| Betta fish | 76–82 °F (24–28 °C) | 6.5–7.5 | 3–12 | 2–8 |
| Goldfish | 65–72 °F (18–22 °C) | 7.0–8.0 | 6–18 | 4–10 |
| African rift lake cichlids (Malawi, Tanganyika) | 75–80 °F (24–27 °C) | 7.8–8.6 | 10–20 | 10–18 |
| Corydoras and bristlenose plecos | 72–78 °F (22–26 °C) | 6.0–7.5 | 2–12 | 2–8 |
| Cherry shrimp (Neocaridina) | 68–78 °F (20–26 °C) | 6.5–8.0 | 6–12 | 2–8 |
| Crystal and bee shrimp (Caridina) | 68–75 °F (20–24 °C) | 5.5–6.8 | 4–6 | 0–1 |
| Planted community tank | 72–78 °F (22–26 °C) | 6.0–7.5 | 4–10 | 2–6 |
Shrimp keepers also track TDS (total dissolved solids). Common targets are about 150–250 ppm for Neocaridina and 100–150 ppm for Caridina kept in remineralized RO water.
03
Ammonia, nitrite and nitrate: the nitrogen cycle numbers
Fish waste and decaying food release ammonia. In a mature tank, bacteria convert it to nitrite and then to nitrate within hours. That is why ammonia and nitrite should always read zero once the tank is cycled, and why nitrate slowly accumulates until you remove it with a water change.
Ammonia toxicity depends on pH and temperature: the same test reading is far more dangerous at pH 8 and 82 °F than at pH 6.5 and 75 °F, because more of it exists as free NH3. Treat any reading above 0.25 ppm as a warning and above about 1 ppm as an emergency. Our guide to ammonia poisoning in fish explains the symptoms, and new tank syndrome covers why it often happens in the first weeks.
Nitrate is far less toxic but still matters. Long exposure above 40 ppm is linked to stress, poor color, stunted growth and algae. If you struggle to keep it down, see how to lower nitrates.
04
pH, GH and KH explained
pH measures how acidic or alkaline the water is, on a log scale: pH 6 is ten times more acidic than pH 7. Most fish tolerate a wide range, but they react badly to sudden changes. If you do need to adjust it, go slowly; see our guides on how to lower pH and how to raise pH.
GH (general hardness) is mainly calcium and magnesium. It affects osmoregulation, shrimp molting and egg development in soft-water species. KH (carbonate hardness) is the water’s buffering capacity: it neutralizes acids and keeps pH stable. With KH below about 2 °dH, acids from waste and CO2 can make pH drop suddenly, a so-called pH crash.
US test kits often report hardness in ppm (mg/L as CaCO3), while European sources and most species profiles use German degrees (°dH). The conversion is simple: 1 °dH equals about 17.9 ppm.
| °dH | ppm (mg/L CaCO3) | Description |
|---|---|---|
| 0–4 | 0–70 | Very soft |
| 4–8 | 70–140 | Soft |
| 8–12 | 140–215 | Medium hard |
| 12–18 | 215–320 | Fairly hard |
| 18–30 | 320–535 | Hard |
| Above 30 | Above 535 | Very hard |
05
Know your tap water first
Before choosing fish, test your tap water once for pH, GH, KH, ammonia and nitrate, or read your local water quality report. Many US cities use chloramine, which a water conditioner neutralizes, and some show a small ammonia reading as a result. Also test tap water left out for 24 hours: pH can rise or fall as dissolved CO2 escapes, which tells you the real stable value.
Working with your tap water is easier and safer than fighting it. Hard, alkaline water suits livebearers, goldfish and rift lake cichlids. Soft, slightly acidic water suits tetras, rasboras and dwarf cichlids. For very soft water species in hard-water areas, keepers usually mix tap water with RO water rather than using chemicals.
06
How to test aquarium water step by step
Liquid reagent kits are more accurate than test strips, especially for ammonia and nitrite. Our best aquarium test kit guide compares the main options.
- Take the sample from mid-water, away from the filter outflow, before a water change.
- Rinse the test tube with tank water and fill it exactly to the line.
- Add reagents in the order and number of drops on the instructions. Shake the nitrate bottles well, as the label requires.
- Wait the full development time, then read the color against a white background in daylight.
- Write down the result with the date and temperature, so you can see trends rather than single values.
- Check reagent expiry dates; expired reagents can give false readings.
07
How often to test
Testing frequency depends on the age and stability of your tank.
| Situation | What to test | How often |
|---|---|---|
| New tank, cycling | Ammonia, nitrite, nitrate, pH | Every 1–2 days |
| First 2–3 months with fish | Ammonia, nitrite, nitrate | Twice a week |
| Established tank | Nitrate, pH | Weekly or every two weeks |
| Established tank | GH, KH | Monthly |
| Sick fish, deaths, cloudy water | Everything, plus temperature | Immediately |
| Shrimp tank | GH, KH, TDS, nitrate | Weekly |
08
Common mistakes
Most water-quality problems come from instability or from misreading results, not from a single value being slightly outside the ideal range.
- Chasing a "perfect" pH with chemicals, creating swings that harm fish more than a stable, slightly off value.
- Relying only on test strips for ammonia and nitrite.
- Ignoring KH in soft water, then facing a sudden pH crash.
- Mixing fish with opposite needs, such as rift lake cichlids and discus.
- Testing only when something looks wrong, with no baseline to compare.
- Assuming clear water is clean water: ammonia and nitrite are invisible. Cloudiness is a separate issue covered in cloudy fish tank water.
09
When to escalate
If ammonia or nitrite is above 0.25 ppm, do a 30–50% water change with conditioned, temperature-matched water and retest the next day. If fish keep showing distress or dying while your readings look normal, have a local fish store test a sample as a cross-check and contact an aquatic veterinarian. Some problems, such as heavy metals or pesticide contamination, do not show on standard hobby kits.
Frequently asked questions
Frequently asked questions
What are the ideal aquarium water parameters?
For most freshwater community tanks: ammonia and nitrite at 0 ppm, nitrate below 20 ppm, pH 6.5–7.5, GH 4–12 °dH, KH 3–8 °dH and a temperature of 76–80 °F (24–27 °C). Specific groups differ: discus and many tetras prefer softer, more acidic water, while livebearers and African cichlids want harder, alkaline water.
What is the difference between pH, GH and KH in an aquarium?
pH measures acidity. GH measures dissolved calcium and magnesium, which affect fish physiology and shrimp molting. KH measures carbonates, the buffer that keeps pH from swinging. In practice, KH controls pH stability, GH matters for species that need soft or hard water, and pH is the result of both plus CO2 and organic acids.
What nitrate level is too high in a fish tank?
Keep nitrate below 20 ppm if possible and below 40 ppm at most. Long exposure above 40–50 ppm stresses fish, fuels algae and can harm sensitive species such as shrimp and discus. Regular water changes, live plants and not overfeeding are the main ways to keep it down.
How often should I test my aquarium water?
Test every one or two days while cycling, twice a week during the first months with fish, then weekly for nitrate and pH once the tank is stable. Check GH and KH monthly. Test everything immediately if fish look sick, die or the water changes appearance.
Is tap water safe for aquarium fish?
Usually yes, once you neutralize chlorine or chloramine with a water conditioner. Test it once to know its pH, hardness and nitrate, then choose fish that suit it. If your tap water is very hard or very soft compared with your fish’s needs, mixing with RO water is safer than adding chemicals.
Sources and references
Written and reviewed by the handsoffmyhobby editorial team according to our editorial policy.
