Aquarium water parameters chart: safe ranges for freshwater

In a typical freshwater community tank, keep ammonia and nitrite at 0 ppm, nitrate under 40 ppm (ideally 20 ppm or less), pH steady between 6.5 and 8.0, GH at 4–8 dGH (70–140 ppm), KH at 4–8 dKH (70–140 ppm) and temperature at 75–80 °F (24–27 °C). Your own fish's species ranges come first, and a steady reading matters more than a perfect one. Test regularly with a liquid kit or strips, and make any change slowly, over days rather than hours.

Ammonia and nitrite have one safe value in every freshwater tank: zero. Nitrate has a ceiling. pH, GH, KH and temperature have ranges that change from species to species, and for those a steady reading matters more than the exact value.

Water parameters chart for freshwater community tanks

These targets suit a typical heated community tank of farm-bred fish. Soft-water specialists and hard-water fish need the species ranges further down.

Parameter	Safe target	Act when	What it tells you
Ammonia (NH3/NH4+)	0 ppm	Any reading above 0	Waste is arriving faster than the filter bacteria can process it
Nitrite (NO2)	0 ppm	Any reading above 0	The filter isn’t fully cycled, or something set it back
Nitrate (NO3)	Under 40 ppm, ideally 20 ppm or less	Above 40 ppm; above 80 ppm, several water changes over a few days	How much waste has built up since the last water change
pH	6.5–8.0, steady	Outside your fish’s range, or lower at every test	How acidic or alkaline the water is
GH (general hardness)	4–8 dGH (70–140 ppm)	Outside your fish’s range; very soft water with snails or shrimp	Calcium and magnesium, the minerals animals build bodies and shells from
KH (carbonate hardness)	4–8 dKH (70–140 ppm)	Below 4 dKH (70 ppm), or lower at every test	Buffering: how well the water resists pH swings
Temperature	75–80 °F (24–27 °C)	Outside your fish’s range, or drifting more than a couple of degrees from your setting	How fast fish eat, grow and use oxygen

Coldwater and temperate fish break the temperature line. White cloud mountain minnows, for example, do best at 61–72 °F (16–22 °C).

What each parameter tells you

Ammonia

Fish excrete ammonia through their gills, and uneaten food adds more. In a cycled tank, filter bacteria convert it quickly, so a test should read 0 ppm. A reading means the tank is still cycling, is overstocked or overfed, or the bacteria were damaged, often by rinsing filter media in tap water. Clear, healthy-looking water can still hold harmful ammonia, so only a test finds it. The ammonia guide covers emergency steps.

Nitrite

Nitrite is the middle step of the nitrogen cycle. It stops the blood carrying oxygen properly, and University of Florida extension guidance says it can be toxic to fish at levels as low as 0.1 ppm. Treat any visible nitrite reading as a problem.

Nitrate

Nitrate is the end product of the cycle. It is far less toxic, so it is allowed to build up between water changes, but months at high levels stress fish, weaken their resistance to disease and feed algae. Keep it under 40 ppm, ideally 20 ppm or less, and lower still for sensitive species and shrimp. Sources disagree on the ceiling (some accept 50 ppm or more), so treat 40 ppm as a conservative limit. These numbers are for kits that report nitrate as NO3, which is how most aquarium kits read. A kit that reports nitrate-nitrogen (NO3-N) shows a figure about 4.4 times lower, so multiply it by 4.4 first. In a well-planted tank, a steady 0 ppm can mean the plants are short of nitrogen.

pH

pH runs from 0 to 14, with 7.0 as neutral. Soft-water fish such as cardinal tetras prefer acidic water below 7; livebearers prefer neutral to alkaline water, 7.0 and up. The scale is logarithmic, so pH 6.0 is ten times more acidic than pH 7.0. pH also shifts through the day as plants and fish change the CO2 in the water, so test at the same time of day.

GH

General hardness measures dissolved calcium and magnesium. Fish need these minerals, shrimp need them to molt and snails need them for shells. Thin or pitted snail shells and shrimp that die while molting can be signs that the water is too soft, though both have other causes too.

KH

Carbonate hardness, close to what chemists call alkalinity, measures carbonate and bicarbonate: the buffer that keeps pH from moving. Filter bacteria use up KH as they work, so it falls between water changes. The Merck Veterinary Manual says alkalinity of 50 ppm (about 3 dKH) or less inhibits the biofilter and advises buffering to at least 100 ppm (about 5.6 dKH). UF/IFAS warns that once alkalinity runs out, pH can drop a long way, fast. Treat 4 dKH as a floor, not a goal.

Temperature

Fish are cold-blooded, so water temperature sets how fast they digest, grow and burn oxygen. Warm water also holds less oxygen, so surface movement matters more at the top of a range. The tank temperature guide lists ranges by species.

Why the same ammonia reading can be harmless or deadly

Your kit reads total ammonia, which is a mix of toxic free ammonia (NH3) and far less harmful ammonium (NH4+). The toxic share rises roughly tenfold for every 1.0 rise in pH, and it also climbs with temperature. This table shows the share at 77 °F (25 °C), worked out from the standard ammonia equilibrium. UF/IFAS’s published factors give 5.0% at pH 8.0 and 75 °F (24 °C), in line with these figures.

pH	Share that is toxic NH3	Free ammonia in a 1 ppm reading
6.5	about 0.2%	0.002 ppm
7.0	about 0.6%	0.006 ppm
7.5	about 1.8%	0.018 ppm
8.0	about 5.4%	0.054 ppm
8.5	about 15%	0.15 ppm

UF/IFAS extension material says fish tissues can be damaged whenever free ammonia is above about 0.05 ppm, with more damage as it rises. So a 1 ppm reading in a pH 8.0 tank is already at the damage line, while the same reading at pH 6.5 is well below it. Two practical lessons follow:

In hard, alkaline water, which is common in livebearer tanks, treat even 0.25 ppm as urgent.
In a neglected tank where pH has slid low, ammonia can sit in its safer form. One huge water change raises the pH and can turn that ammonia toxic. The Merck Veterinary Manual’s advice for this “old tank syndrome” is small water changes every day instead. Changing 10–25% a day keeps each pH step small.

Species ranges: check your fish, not just the chart

The community chart is a middle ground. Every species has its own window, and two fish that both “do fine in tap water” can have ranges that barely touch. These ranges come from FishFits’ species pages.

Species	Temperature	pH	GH (dGH)
Cardinal tetra	73–82 °F (23–28 °C)	4.5–7.0	1–8
Neon tetra	70–77 °F (21–25 °C)	5.5–7.5	1–10
Harlequin rasbora	72–81 °F (22–27 °C)	6.0–7.5	2–12
Bronze corydoras	70–79 °F (21–26 °C)	6.0–8.0	2–15
Betta	75–82 °F (24–28 °C)	6.0–8.0	2–15
Dwarf gourami	75–82 °F (24–28 °C)	6.0–7.5	4–15
Zebra danio	64–77 °F (18–25 °C)	6.5–8.0	5–18
White cloud mountain minnow	61–72 °F (16–22 °C)	6.5–8.0	5–18
Guppy	72–81 °F (22–27 °C)	7.0–8.0	8–20
Platy	68–79 °F (20–26 °C)	7.0–8.0	10–25
Molly	75–81 °F (24–27 °C)	7.5–8.5	15–30

Read the table as a set. Neon tetras and mollies overlap only at 75–77 °F and pH 7.5, and their GH ranges don’t meet at all (neons top out at 10 dGH, mollies start at 15). One of them will always live outside its comfort zone. Neon tetras and bronze corydoras, by contrast, share 70–77 °F (21–25 °C), pH 6.0–7.5 and 2–10 dGH.

Every species in the fish database lists its range with sources, and the stocking calculator works out the shared temperature, pH and GH window for your whole stock list. Then compare that window with your tap water. If the tap already sits inside it, you have nothing to adjust.

GH and KH: degrees, ppm and what “soft” means

Kits report hardness either in German degrees (dGH, dKH) or in ppm (mg/L as calcium carbonate). One degree is about 17.8 ppm (17.848 exactly), and the unit converter switches between them. The bands below follow the USGS hardness scale.

Water	ppm (mg/L)	Degrees	Fish that like it
Soft	0–60	0–3.4	Cardinal tetras, dwarf rasboras, green neon tetras
Moderately hard	61–120	3.4–6.7	Most farm-bred community fish
Hard	121–180	6.8–10.1	Guppies, zebra danios
Very hard	over 180	over 10.1	Platies, mollies

GH and KH are separate readings. Baking soda raises KH but not GH, and some mineral products raise GH without touching KH, so test both.

Test kits: liquid vs strips

	Liquid (drop) kits	Test strips
Speed	Up to about 5 minutes per test, one parameter per tube	Under a minute for several readings at once
Coverage	Separate kits for ammonia, nitrite, nitrate, pH, GH and KH	Many multi-test strips leave out ammonia, so it needs its own test
Reading the color	A full tube makes close shades easier to compare	Close shades on small pads are harder to tell apart
Weak spots	Nitrate reagent not shaken hard enough; reading too early or too late	Damp strips; touched pads; reading after the stated time
Shelf life	Reagents expire, so check the date	Spoil quickly if the container is left open
Best for	Cycling, sick fish, double-checking an odd strip result	Routine checks of nitrate, pH, GH and KH

GH and KH drop kits work by counting drops until the color changes. With most kits each drop equals 1 degree, but check your kit’s instructions.

How to get a reading you can trust:

Rinse the tube with tank water, then fill it exactly to the line.
Follow the drop count, shaking and wait time on the instructions to the second. Liquid ammonia tests often need about 5 minutes for the color to develop.
Shake the nitrate reagent bottle very hard before use. Undershaking is the classic cause of false-low nitrate readings.
Read the color in daylight against a white background.
Don’t pour test water back into the tank.
Test your tap water once for a baseline. Many taps carry some nitrate, and chloramine-treated water can show ammonia. For tap pH, let a glass stand for 24 hours first, because dissolved CO2 can make fresh tap water read lower than it will in the tank.

How often to test

Situation	What to test	How often
Cycling a new tank	Ammonia, nitrite, nitrate, pH	Daily or every other day
First 4–6 weeks with fish	Ammonia, nitrite, nitrate	Several times a week
Established tank	Nitrate	Every 1–2 weeks
Established tank	pH, KH, GH	About monthly
Every day	Temperature	A quick look at the thermometer
New fish, new food, after a vacation	Ammonia, nitrite	In the few days that follow
Fish acting oddly, or a death	Everything, compared with your log	Right away

How to adjust each parameter safely

The rule for all of them: change slowly. Fish cope far better with a reading that is a little off than with a sudden swing toward a “perfect” number. For the same reason, skip pH up and pH down chemicals: pH jumps, then often drifts back within a day or two, and each round is another swing.

Ammonia or nitrite above zero

Feed very lightly or not at all for 1–2 days. Fry still need small meals.
Change 25–50% of the water with dechlorinated water within about 2 °F (1 °C) of the tank.
Retest the next day and repeat until both read 0. In a long-neglected tank with low pH, change only 10–25% a day instead, so the pH rises slowly and the stored ammonia doesn’t suddenly turn toxic.
Find the cause: a new tank, a big batch of new fish, overfeeding, a dead fish, a filter rinsed in tap water or a recent medication.
Ammonia-binding water conditioners buy time, but they don’t remove the cause. Dose them by the label.
For nitrite, chloride reduces how much the gills take up, which is why the Merck Veterinary Manual lists adding chloride as part of treatment and aquarium salt is sometimes suggested. Salt doesn’t remove nitrite, so keep changing water. Follow the label, and check that your fish, plants and snails tolerate salt first.
If fish are gasping at the surface, or still look unwell once both readings are back to 0, contact an aquatic vet.

Nitrate too high

A water change removes about its own percentage of nitrate, so changing 50% halves the reading if your tap water has none. The water change calculator shows how many changes it takes to reach a target such as 40 ppm and where nitrate settles on your weekly routine. Above 80 ppm, split the work into several changes over a few days. Long term, feed less, add fast-growing plants or keep fewer fish.

pH too low or drifting down

A falling pH usually means KH has run out, so fix KH and pH follows. If the tank has any ammonia, deal with that first, because a rising pH makes ammonia more toxic. Crushed coral is the gentlest fix: about 1 lb per 10 gallons (0.45 kg per 38 L) mixed into the substrate is a common starting point, or put it in a mesh bag in the filter. It dissolves faster when pH is low and slows as pH rises, so it works gradually, and it needs topping up or replacing every 6–12 months as it is used up. More frequent water changes also work if your tap water’s pH and KH are fine.

pH too high for soft-water fish

Mix reverse osmosis (RO) water with tap water, raising the RO share a little at each water change. Driftwood, peat and dried leaves can nudge pH down in soft, low-KH water but barely move hard water. Often the simplest answer is fish that suit your tap.

GH and KH

Raise both: crushed coral or a GH-and-KH mineral supplement, following the label.
Raise KH only: baking soda (sodium bicarbonate). A level 1/4 teaspoon (about 1.2 g) per 10 gallons (38 L) raises KH by roughly 1 dKH. Dissolve it in a cup of tank water, add no more than that in a day and retest. It raises pH as well, so clear any ammonia first.
Raise GH only: a GH mineral supplement for shrimp or livebearers, following the label.
Lower both: blend in RO water. Don’t use water from a household softener. It swaps calcium and magnesium for sodium, so GH reads low while KH and pH stay high, and it isn’t the soft water that soft-water fish come from.

Temperature

Use a heater sized for the tank and a separate thermometer, and move the setting no more than about 2 °F (1 °C) a day.

Common mistakes

Chasing a perfect pH. A steady 7.6 is better than a 6.8 that swings every week.
One giant water change in a neglected tank. The sudden pH rise can turn hidden ammonia toxic.
Splitting the difference between soft-water and hard-water fish, so neither group is comfortable.
Trusting old or damp tests. Expired reagents, open strip containers and an unshaken nitrate bottle all give false readings.
Testing only when something is wrong. Without a baseline, you can’t tell what changed.

Keep a log so you can see trends

One reading tells you little, but a run of readings gives early warnings. A KH that falls each week warns of a coming pH crash. Nitrate that climbs faster than usual points to overfeeding or a dead fish hidden in the plants.

My tanks is a free test log for ammonia, nitrite, nitrate, pH, temperature, GH and KH. It charts ammonia, nitrite and nitrate over time, shows the temperature, pH and GH range your saved fish share, and keeps everything in your browser with no account. Each time, note the date, every reading, the size of any water change, and anything new such as fish, food or medication. That history is also useful to an aquatic vet if your fish get sick.

Gear mentioned in this guide

Liquid test kit (ammonia, nitrite, nitrate, pH) (opens Amazon in a new tab)Liquid kits are more accurate than strips. The API Freshwater Master Test Kit is the common choice. How to choose
GH and KH test kit (opens Amazon in a new tab)Measures hardness and buffering, which matter for soft-water and rift lake fish. How to choose
Test strips for quick checks (opens Amazon in a new tab)Handy for a fast look, but confirm problems with a liquid kit. How to choose

Product links go to Amazon search results. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. Disclosure.

Frequently asked questions

What is the most important water parameter in a fish tank?

Ammonia and nitrite. Both should read 0 ppm in a cycled tank, and any reading means waste is building up faster than the filter can handle it. After those, a stable pH and temperature matter more than hitting an exact number.

Are aquarium test strips accurate enough?

Strips are fine for routine checks of nitrate, pH, GH and KH if they are in date, kept dry and read at the time the label gives. Their color steps are coarse, and many multi-test strips don't measure ammonia at all, so use a liquid kit for ammonia and nitrite and to double-check an odd result, especially while cycling or when fish are sick.

What should GH and KH be in a community tank?

About 4–8 dGH and 4–8 dKH (70–140 ppm each) suits most farm-bred community fish. Don't let KH fall to about 3 dKH (54 ppm) or lower, because the filter bacteria start to struggle and pH can crash. Livebearers want harder water and soft-water fish such as cardinal tetras want softer.

How often should I test aquarium water?

Daily or every other day while cycling, several times a week for the first 4–6 weeks with fish, then nitrate every week or two and pH and KH about monthly. Test everything straight away if fish act oddly or one dies.

Can I just keep my fish at my tap water pH?

Usually, yes. Most farm-bred community fish adapt to a steady pH between about 6.5 and 8.0, and a pH you never fight is safer than a 'perfect' one that swings. Soft-water specialists such as dwarf rasboras and hard-water fish such as mollies are the exceptions.