How often should I change the water?

There is no single interval valid for every aquarium, because it depends on the volume, the number of fish and the organic load. The practical rule is to prefer regular partial changes over a large occasional one, and to let the nitrate tests tell you whether the current cadence is enough or needs increasing. Consistency matters more than the size of any single change.

Why should I not change all the water at once?

A total change also removes much of the beneficial bacteria and causes an abrupt swing in parameters, both of which stress the fish and can reset the nitrogen cycle. Partial changes remove accumulated waste while keeping the environment stable and preserving the bacterial colonies that live in the filter and substrate.

Can I clean the filter under the tap?

No, tap water contains chlorine that kills the beneficial bacteria colonised in the filter, and you would restart the cycle from scratch. Rinse the filter media in water taken from the tank during the change, gently, and replace them only when they are genuinely worn out, and never all at the same time.

My fish are gasping at the surface: what do I do?

Gasping at the surface often points to low oxygen or altered parameters. As a first step measure ammonia, nitrite and temperature and do a partial change with treated water at the right temperature. If the behaviour continues despite correct values, or if other signs appear, consult a vet experienced with aquatic animals.

I just set up the tank: how many fish can I add?

In a freshly set up tank the nitrogen cycle is not yet mature, so you start with very few fish and confirm with tests that ammonia and nitrite stay undetectable before increasing the population. Filling the tank straight away is the mistake that causes the most deaths among beginners: patience in the first weeks is the most important part.

Non conventional species30 minLevel: IntermediateUpdated on August 5, 2026

Keeping the water quality right in your aquarium

How to measure ammonia, nitrite and nitrate, understand the nitrogen cycle and do regular partial water changes to keep the water healthy and the fish well.

Audience: Pet owners
Species: Fish
Scope: Valid everywhere

What you need before you start

Un kit di test dell'acqua a reagenti liquidi per ammoniaca, nitriti e nitrati, più un termometro affidabile
Un secchio riservato solo all'acquario e un tubo per il sifonamento del fondo
Un biocondizionatore per rendere sicura l'acqua di rubinetto prima di aggiungerla
Un quaderno o una nota sul telefono per registrare misure, cambi e osservazioni

For a fish, water is not the setting it lives in: it is the air it breathes, the place where it eats and where it excretes its waste. That is why most aquarium problems do not come from mysterious diseases but from poorly managed water, and the most effective care is almost always prevention through water quality. At the heart of it all is the nitrogen cycle, the process by which beneficial bacteria turn the most toxic nitrogen waste into less dangerous compounds, which are then removed with water changes. This guide does not give you numbers to hit, because those depend on the species you keep and must be checked for your case, but it teaches the method, measure regularly, understand what the numbers say and act with well done partial changes. It is half an hour of weekly work that keeps fish healthy far better than any medicine bought after a problem has already broken out.

The nitrogen cycle in plain words

Fish and food residues produce ammonia, which is very toxic. In a mature aquarium, colonies of beneficial bacteria turn ammonia into nitrite, also toxic, and then nitrite into nitrate, far less dangerous. Nitrate builds up and is lowered by water changes. This process, the nitrogen cycle, establishes over a few weeks when the aquarium is new, and that is why you do not stock a freshly set up tank: without mature bacteria, ammonia rises and poisons the fish. Understanding this chain explains almost everything that happens in the tank.

What you measure and what it tells you
Parameter	What it represents	Rough reading
Ammonia	Most toxic waste, produced continuously	Should read undetectable in a mature tank
Nitrite	Intermediate product, still toxic	Should read undetectable in a mature tank
Nitrate	Final product, far less toxic	Builds up: controlled with water changes
Temperature	Affects metabolism and oxygen	Kept stable within the species range

Measuring parameters with method

Measuring is not an occasional act done when a fish is sick: it is a routine that captures the tank while it is healthy, so you can spot a drift before it becomes a problem. Liquid reagent kits are generally more reliable than strips, and consistency matters more than absolute precision.

Always measure at the same time and with the tank in the same conditions, so the numbers are comparable over time.
In the first weeks of a new tank check ammonia and nitrite often, because that is the phase when they can rise.
In a mature tank keep an eye above all on nitrate, which shows how loaded the system is.
Note each measurement with the date: it is the trend over time, not the single number, that tells the tank's story.
If a value surprises you, repeat the measurement before acting: a reading error happens, and a drastic response to a wrong figure does harm.

The partial water change, step by step

Prepare the bucket and new water
Use a bucket kept only for the aquarium, never one that has held detergents. Prepare the new water at a temperature close to the tank's, so you do not give the fish a thermal shock.
Treat the tap water
Add the water conditioner as the product directs, to neutralise chlorine and chloramine, which harm both fish and beneficial bacteria. Never pour untreated tap water straight into the tank.
Siphon the substrate, removing part of the water
With the tube vacuum debris from the bottom while removing only a partial share of the water, not all of it. A partial change removes accumulated waste without wiping out the bacterial population that also lives in the substrate.
Reintroduce the new water gently
Pour the treated water slowly, without dumping it onto the fish or stirring the bottom too much. A gradual top up keeps parameters stable and reduces stress.
Clean the filter without sterilising it
If the filter needs cleaning, rinse the media in water taken from the tank, not under the tap. Tap water kills the beneficial bacteria that colonise the filter, and you would start the cycle over.
Record the change, amount and observations
Note when you did the change, how much water you replaced and how the fish behave afterwards. A log makes visible what works and helps you keep a regular cadence.

Feeding and overstocking

Two management mistakes can overwhelm even a well cycled tank, excess food and overstocking. Uneaten food decomposes and becomes ammonia, and too many fish produce more waste than the bacteria and the changes can process. Reducing these two factors solves many water quality problems without adding anything.

Give amounts the fish finish quickly and remove leftovers: food left on the bottom turns into ammonia.
Do not add fish to fill the tank: the population must stay within what the system can process.
Introduce a few fish at a time, so the bacterial colonies have time to adjust to the larger load.
Remember that fish grow: a tank suitable for small specimens can become cramped when they reach adult size.
Before adding new fish, learn about the species' compatibility and space needs, to avoid tension and overload.

Signs that call for a check

Sign, possible cause and first response
Sign	Possible cause	First response
Fish gasping at the surface	Low oxygen or altered parameters	Measure the values and do a partial change, then assess
Suddenly cloudy water	Bacterial bloom or excess food	Reduce the food and check ammonia and nitrite
Listless fish on the bottom or with clamped fins	Environmental stress or water quality	Measure parameters before assuming disease
Detectable ammonia or nitrite	Immature cycle or overload	Partial change and less food, then re measure
Body marks, spots or abnormal behaviour	Possible disease	Consult a vet experienced with aquatic animals

Many symptoms that look like disease are in fact the result of altered water, which is why the first thing to do when fish struggle is to measure the parameters, not to add products at random. If the values are correct and the fish stay unwell, or if you see lesions and abnormal behaviour, turn to a vet experienced with aquatic animals, because not every area has one and it is worth finding them before you need them.

Frequently asked questions

How often should I change the water?: There is no single interval valid for every aquarium, because it depends on the volume, the number of fish and the organic load. The practical rule is to prefer regular partial changes over a large occasional one, and to let the nitrate tests tell you whether the current cadence is enough or needs increasing. Consistency matters more than the size of any single change.
Why should I not change all the water at once?: A total change also removes much of the beneficial bacteria and causes an abrupt swing in parameters, both of which stress the fish and can reset the nitrogen cycle. Partial changes remove accumulated waste while keeping the environment stable and preserving the bacterial colonies that live in the filter and substrate.
Can I clean the filter under the tap?: No, tap water contains chlorine that kills the beneficial bacteria colonised in the filter, and you would restart the cycle from scratch. Rinse the filter media in water taken from the tank during the change, gently, and replace them only when they are genuinely worn out, and never all at the same time.
My fish are gasping at the surface: what do I do?: Gasping at the surface often points to low oxygen or altered parameters. As a first step measure ammonia, nitrite and temperature and do a partial change with treated water at the right temperature. If the behaviour continues despite correct values, or if other signs appear, consult a vet experienced with aquatic animals.
I just set up the tank: how many fish can I add?: In a freshly set up tank the nitrogen cycle is not yet mature, so you start with very few fish and confirm with tests that ammonia and nitrite stay undetectable before increasing the population. Filling the tank straight away is the mistake that causes the most deaths among beginners: patience in the first weeks is the most important part.

What to do next

For fish, health is mostly water quality: measure ammonia, nitrite and nitrate regularly, understand the nitrogen cycle and maintain the tank with frequent partial changes, always treating the tap water and never sterilising the filter. Do not stock an uncycled tank, avoid excess food and overstocking, and when fish struggle, measure the parameters before assuming disease, turning to a vet experienced with aquatic animals when needed.