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Aquarium water parameters explained

What the numbers on your test kit actually mean, what causes each parameter to drift, and the ranges to aim for in a healthy freshwater tank.

Published 6 June 2026 · Last updated 6 June 2026 · 14 min read

What "water parameters" actually means

"Water parameters" is shorthand for the measurable chemical and physical properties of the water in your tank. Healthy fish depend on these values staying within ranges their species can tolerate, and on the values being stable rather than swinging up and down.

There are two truths about parameters that matter more than any specific number:

Stability matters more than perfection. A tank steady at pH 7.8 is healthier than a tank bouncing between pH 6.8 and 7.4, even though the second tank's range includes "better" values. Fish acclimate to consistent parameters over weeks; sudden shifts cause stress, illness, and death even when the new values are technically "ideal."

Your tap water is your starting point. Every parameter conversation has to begin with what comes out of your tap, because you'll be replacing 25% of your water with it every week. Species that don't match your tap water are species you'll be fighting your tap water to keep, every water change, for the life of the tank. Choosing species suited to your local water is the single biggest decision a beginner can make.

How to test, and how often

You need a liquid freshwater test kit, not test strips. Strips are imprecise — they read ranges rather than specific values, and the colour-matching is unreliable at the low concentrations that matter most. The API Freshwater Master Test Kit is the most widely-used reference in the hobby and stocks in almost every Australian aquarium shop. It covers pH, ammonia, nitrite, and nitrate. Hardness (GH and KH) test kits are sold separately.

For a stable, established tank, test:

For a new, uncycled tank, test ammonia, nitrite, and nitrate daily until the cycle is established — see our cycling guide for full detail.

Temperature

The single easiest parameter to control and one of the most important. Tropical freshwater species need warm, stable water; cold-water species need cold, stable water; and mixing the two is rarely successful.

Typical target ranges:

The mistake beginners make isn't usually picking the wrong number — it's letting temperature swing. A heater that cycles between 23°C and 27°C across a day is more stressful for fish than a heater steadily holding 25°C, even though both averages are the same. A good thermostat-controlled heater and a thermometer mounted on the opposite side of the tank from the heater (so you measure water that's actually circulated through, not the warm spot beside the element) keeps things stable.

For sizing a heater to your tank, use our heater wattage calculator.

pH

pH measures how acidic or alkaline your water is on a scale from 0 to 14.

Most freshwater fish live in the range 6.0–8.0 in the wild, and the species you keep should match where they're from in this range.

Importantly, pH is a logarithmic scale: pH 6 is ten times more acidic than pH 7, and pH 5 is a hundred times more acidic than pH 7. Small numerical changes represent large chemical shifts.

Typical target ranges:

pH is strongly linked to your tank's buffering capacity (see the KH section below). Low KH tanks can have their pH crash rapidly as nitric acid builds up between water changes — a phenomenon known as "old tank syndrome." High KH tanks resist pH changes more strongly. This is why some keepers find their pH stubbornly refuses to budge and others find it bouncing every week.

Trying to chemically adjust your tap water's pH is generally a beginner mistake. The "pH down" and "pH up" products at pet shops produce unstable adjustments that swing back within days, often crashing parameters in the process. Choosing species that match your tap water is almost always better than fighting it.

Ammonia (NH3/NH4+)

Ammonia is the most acutely toxic of the common parameters and the one that kills fish fastest. It enters the tank as fish waste, decomposing food, and decaying plant matter, and is converted by beneficial bacteria into nitrite (the next parameter down).

Target: 0 ppm, always, in a cycled tank.

Any detectable ammonia in an established tank indicates something is wrong: a fish has died and is decomposing, the filter has crashed, you've overstocked, the filter media has been over-cleaned, or you've added too many fish at once.

Ammonia toxicity depends heavily on pH and temperature. In acidic water, most ammonia exists as the relatively non-toxic ammonium ion (NH4+). In alkaline water, more of it exists as the highly toxic free ammonia (NH3). This means the same 1 ppm ammonia reading is dramatically more dangerous at pH 8.0 than at pH 6.5. Australian tap water's alkalinity makes ammonia management slightly more critical here than in soft-water regions.

If you find ammonia in your tank, the immediate response is a large water change (50%+) with properly dechlorinated water. Seachem Prime dosed at 5x the normal rate also temporarily neutralises ammonia for around 24 hours, buying you time to find and fix the underlying cause.

Nitrite (NO2-)

Nitrite is the second stage of the nitrogen cycle — beneficial bacteria convert ammonia to nitrite, then a different group of bacteria convert nitrite to nitrate. Nitrite is also acutely toxic, though slightly less so than ammonia.

Target: 0 ppm, always, in a cycled tank.

Nitrite causes "brown blood disease" — it binds to fish haemoglobin and prevents oxygen transport, slowly suffocating fish even in well-aerated water. Signs include fish gasping at the surface, listlessness, and red or brown gill colouration.

Any nitrite reading above zero in an established tank indicates the same problems as detectable ammonia, usually meaning the cycle has been disrupted. The fix is the same: large water change, dose Prime, identify the cause (often filter cleaning in chlorinated tap water, a power outage, or massive overstocking).

A small dose of aquarium salt (1 tablespoon per 20 litres) is sometimes used short-term to reduce nitrite toxicity to fish during a crisis — the chloride competes with nitrite uptake at the gills. This is an emergency measure only, and is not appropriate for shrimp or certain catfish.

Nitrate (NO3-)

Nitrate is the final stage of the nitrogen cycle and the only one that's relatively safe at low concentrations. It accumulates in the tank until you export it via water changes (or via live plants, which consume it as fertiliser).

Target ranges:

If your nitrate keeps rising despite regular water changes, your options are: increase the frequency or size of water changes, add live plants, reduce stocking, or reduce feeding. Note also that some Australian tap water has detectable nitrate (typically 5–15 ppm), particularly in agricultural areas. You can't get below your tap water's nitrate level with water changes alone — only plants or reverse osmosis (RO) water remove what's already there.

General hardness (GH)

GH measures the concentration of dissolved minerals — primarily calcium and magnesium — in your water. It's what makes water "hard" or "soft" in the kitchen sense.

GH is measured in parts per million (ppm) or degrees of general hardness (dGH). The conversion is 1 dGH ≈ 17.9 ppm. Australian water is most commonly reported in ppm; European hobby resources often use dGH.

Categories:

GH affects fish in two ways. Functionally, soft-water species are adapted to low-mineral water and struggle to osmotically regulate in mineral-rich water (and vice versa). Behaviourally, fish from particular regions display best colour, breed most readily, and live longest in water that matches their natural conditions.

Australian tap water is typically hard, which significantly affects species selection — see the Australian section below.

Carbonate hardness (KH)

KH measures the carbonate and bicarbonate concentration in your water — the chemicals that buffer pH against change. High KH = stable pH that resists swings. Low KH = pH that can drift or crash easily.

KH is also typically measured in ppm or in degrees (dKH), with the same conversion ratio as GH: 1 dKH ≈ 17.9 ppm.

Functional ranges:

KH and pH are linked. As KH decreases (typical in tanks with infrequent water changes), the water's buffering capacity erodes and pH starts to drift downward. This is the most common cause of "old tank syndrome" — gradual parameter decline that fish appear to tolerate until they suddenly don't.

Regular water changes restore KH from your tap water's baseline. If your tap water itself is very low in KH, you may need to supplement with crushed coral, aragonite, or a commercial KH buffer to maintain stability.

Dissolved oxygen

Fish breathe oxygen from water via their gills, and tank inhabitants — including beneficial bacteria — consume oxygen continuously. Most well-maintained community tanks have adequate dissolved oxygen without any specific intervention, but several factors can deplete it dangerously fast.

You generally don't measure dissolved oxygen directly. Instead, watch for the warning signs of low oxygen:

Things that reduce dissolved oxygen:

The simplest fix for low oxygen is surface agitation: angle your filter return upward so it ripples the surface, or add an air stone. CO2-injected planted tanks need careful balance between CO2 and oxygen, particularly overnight when CO2 should be cut off.

Australian tap water — what to expect

Australian tap water varies significantly by region, but a few general patterns affect species selection across most of the country:

South East Queensland (Brisbane, Gold Coast, Sunshine Coast): typically hard to very hard (GH 120–180 ppm) and alkaline (pH 7.5–8.0), with moderate to high KH. Excellent for livebearers, snails, African cichlids, and most hard-water species. Difficult for soft-water tetras, discus, and shrimp without modification.

Sydney and surrounds: moderate hardness (GH 50–100 ppm), slightly alkaline pH (around 7.5). The most flexible Australian water — supports a broad species range without much modification.

Melbourne: notably soft (GH around 10–30 ppm) and slightly acidic to neutral. Excellent for soft-water community species, tetras, and discus. Challenging for livebearers and snails without supplementation.

Adelaide: hard and alkaline, similar profile to South East Queensland.

Perth: very hard tap water in most areas. Best for hard-water species.

Chloramine is now used as the primary disinfectant in most Australian municipal supplies, which means dechlorinator is non-negotiable — chlorine evaporates if water is left standing, but chloramine doesn't. Seachem Prime handles both chlorine and chloramine and is the most widely-used dechlorinator in Australian aquarium keeping.

Your specific local profile is published by your water authority. SEQ Water publishes monthly water quality reports. For a quick test of your specific situation, fill a clean glass jar from the tap and test pH, GH, and KH after letting it sit for a few hours.

Troubleshooting common parameter problems

My pH keeps dropping

Almost always a KH problem. Test your KH — if it's low, the water has no buffering capacity and is acidifying as nitric acid accumulates between water changes. Fix: larger or more frequent water changes to restore KH from your tap water, or supplement with crushed coral in the filter to add carbonate buffering.

My nitrate won't go down despite weekly water changes

Two possibilities. Your tap water might have detectable nitrate already (test it directly — fill a glass from the tap, age it briefly, then test). Or your stocking and feeding are producing nitrate faster than weekly 25% changes export. Options: larger water changes, more plants, less food, less stocking.

I have ammonia readings in a tank that's been cycled for months

Something disrupted the cycle. Common causes: filter media cleaned in tap water (kills bacteria), a fish died and is decomposing somewhere out of sight, medication was added (some antibiotics destroy the nitrogen cycle), or stocking was increased too quickly. Immediate response: large water change, dose Seachem Prime, find and remove any dead fish, leave the filter alone.

My fish are gasping at the surface every morning

Low dissolved oxygen overnight. In planted tanks, plants consume oxygen at night; in heavily stocked tanks, the bioload outpaces oxygen production. Add surface agitation (angle filter output up, or add an air stone running on a timer overnight).

My GH and KH are very different from each other

This is normal. GH measures total dissolved minerals; KH measures only the carbonate and bicarbonate fraction. Water can be high in GH but low in KH (common with some bore water sources), which means hard water that still has unstable pH. Treat the KH separately by supplementing with carbonate buffer if needed.

I added pH-adjusting chemicals and now everything is worse

Don't continue adding more. Most pH-adjusting products work by adding acids or bases that get consumed within days, causing rebound swings. Stop dosing, do several 25% water changes over the next week to restore your tap-water baseline, and choose species suited to that baseline rather than fighting it.

For complete tank setup from scratch, see how to set up a new aquarium. For choosing fish suited to your water, see how to choose your first fish. For ongoing maintenance, see water changes. If your fish look unwell, see common fish diseases.

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