How to Set Up a Freshwater Aquarium

Setting up a freshwater aquarium is an exciting project, but creating a healthy home for fish involves more than simply filling a tank with water. The aquarium needs suitable equipment, stable water conditions and a mature biological filter before fish can safely be introduced.

Taking your time during the setup process will help prevent poor water quality, stressed fish and many of the problems commonly experienced with new aquariums.

This guide explains how to plan, assemble and prepare a freshwater aquarium properly.

Before You Choose Your Aquarium

It is best to decide which fish you would like to keep before buying your aquarium. Different species have different requirements, including:

• Minimum aquarium size
• Preferred water temperature
• Suitable pH and water hardness
• Adult size
• Swimming space
• Group size
• Compatibility with other species
• Filtration and water-flow requirements

Always research the adult size of each fish rather than choosing based on its size in the shop. Some small juvenile fish can grow surprisingly large.

Many fish also need to live in groups. Schooling and shoaling species should not be kept alone simply because the aquarium does not have room for a suitable group.

A larger aquarium is often easier to maintain than a very small one because the greater volume of water is less vulnerable to rapid changes in temperature and water quality.

What You Will Need

The exact equipment will depend on the fish and plants you intend to keep, but most freshwater aquariums require:

• A suitably sized aquarium
• A purpose-made aquarium cabinet or strong, level surface
• A secure lid or cover
• An appropriately sized aquarium filter
• A thermostatically controlled heater for tropical species
• An aquarium thermometer
• Aquarium lighting
• Substrate, such as aquarium gravel, sand or planted-aquarium soil
• Aquarium-safe rocks, wood, ornaments and hiding places
• Live or artificial aquarium plants
• Tap water conditioner
• A reliable aquarium water-testing kit
• A bacterial starter or mature, disease-free filter media
• A suitable ammonia source for fishless cycling
• A clean bucket used only for aquarium maintenance
• A gravel cleaner or aquarium siphon
• An algae pad or aquarium-safe glass cleaner
• A fish net
• Suitable food for the fish you plan to keep

Never use household soap, disinfectant or cleaning spray on anything that will come into contact with the aquarium.

Choosing the Right Location

Choose the aquarium’s permanent position before filling it. Once filled, an aquarium becomes extremely heavy and should never be moved with water, substrate or decorations inside.

Place the aquarium:

• On a level, stable surface designed to support its filled weight
• Away from direct sunlight
• Away from radiators, fires and draughts
• Near suitable electrical sockets
• Somewhere that allows access for cleaning and maintenance
• Away from cigarette smoke, aerosols and household chemicals
• In a relatively quiet area without constant knocks or vibration

Direct sunlight can encourage excessive algae growth and may cause the water temperature to rise. Leave enough space around the aquarium to reach the filter, cables and pipework.

Electrical cables should form a drip loop below the socket so that water cannot run directly towards the plug.

Step-by-Step Aquarium Setup

1. Check the Aquarium and Cabinet

Inspect the aquarium for chips, cracks or damaged seals. Make sure the cabinet or supporting surface is level and suitable for the aquarium’s weight.

Follow the aquarium manufacturer’s instructions regarding the use of a support mat. Some aquariums require one, while others are designed to sit directly on their matching cabinet.

Never place a filled aquarium on an unsuitable item of furniture.

2. Rinse the Aquarium and Equipment

Wipe the inside of the aquarium with clean water and a new aquarium-safe cloth or sponge.

Rinse gravel, rocks, artificial plants and ornaments using clean water unless the product instructions state otherwise. Do not use soap, washing-up liquid or household cleaning products.

Some specialist substrates and planted-aquarium soils should not be rinsed, so always check the packaging.

Natural rocks and wood must be suitable for aquarium use. Some materials can alter the pH or hardness of the water, while collected items may carry pollutants, parasites or unwanted organisms.

3. Add the Substrate

Spread the chosen substrate evenly across the bottom of the aquarium.

A depth of approximately 3–5cm is suitable for many standard gravel or sand aquariums, although planted aquariums may require a different depth or specialist substrate.

You can create a gentle slope, with slightly more substrate towards the back, to add depth to the display and help debris collect towards the front for easier cleaning.

4. Position Decorations and Plants

Add rocks, wood, caves and other decorations before filling the aquarium completely.

Make sure that:

• Heavy decorations are stable and cannot topple
• Rocks are not resting insecurely on loose substrate
• Fish cannot become trapped inside ornaments
• All openings are large enough for the intended fish
• Sharp or rough edges will not damage fins or bodies
• There is still enough open swimming space

Provide hiding places and sheltered areas. These can help fish feel secure and reduce stress, especially when they are first introduced.

If you are using live plants, place them according to their eventual height and spread. Taller plants usually work well towards the back, with shorter plants at the front.

Not every aquarium plant should be buried. Plants such as Anubias and Java fern have a rhizome that should normally remain above the substrate and can instead be attached to wood or rock.

5. Install the Filter

Install the filter according to the manufacturer’s instructions.

The filter must be appropriate for:

• The aquarium’s water volume
• The number and type of fish
• The amount of waste the fish are likely to produce
• The level of water movement the fish prefer

A filter performs several important jobs:

• Mechanical filtration traps visible particles.
• Biological filtration supports beneficial bacteria that process harmful fish waste.
• Chemical filtration uses specialist media to remove particular substances when required.

Do not switch the filter on until the aquarium has been filled and the filter has been correctly primed. Some filters must be filled with water before starting.

The filter will need to run continuously, 24 hours a day. Switching it off for long periods reduces oxygen reaching the beneficial bacteria and may damage the biological filter.

6. Install the Heater and Thermometer

Tropical freshwater fish generally require a thermostatically controlled aquarium heater.

Position the heater where water can circulate around it, ideally near the filter flow, and follow the manufacturer’s instructions carefully.

Allow a newly submerged heater to stand in the water for the specified period before switching it on. This allows its temperature to equalise and reduces the risk of damage.

Set the heater to the temperature required by the fish you plan to keep. There is no single correct temperature for every tropical species.

Use a separate thermometer to check the actual water temperature. Do not rely entirely on the setting printed on the heater.

Most aquarium heaters should be switched off and allowed to cool before a water change exposes them to the air.

7. Fill the Aquarium

Place a clean plate, bowl or plastic bag on top of the substrate and pour the water slowly onto it. This will reduce disturbance and prevent the substrate from being scattered.

Treat tap water with a suitable water conditioner. Tap water can contain chlorine or chloramine, which may harm fish and the beneficial bacteria in the filter.

Follow the water conditioner’s dosing instructions carefully. If the product dose is based on the aquarium volume, remember that substrate and decorations reduce the amount of water the aquarium actually holds.

Fill the aquarium to the appropriate level, leaving any space required by the filter or lid.

8. Start the Equipment

Once the aquarium is filled:

1. Check that the heater is fully submerged as instructed.
2. Prime the filter if required.
3. Switch on the filter.
4. Switch on the heater after the recommended resting period.
5. Check for leaks.
6. Make sure water is circulating properly.
7. Confirm the temperature using the thermometer.

Cloudiness caused by disturbed substrate may settle or be removed by the filter. New wood can sometimes release tannins, giving the water a tea-coloured appearance. Tannins are not normally dangerous to suitable freshwater fish, although they may alter the appearance and chemistry of the water.

9. Set the Lighting Period

Use a timer to provide a regular day-and-night cycle.

Around six to eight hours of light per day is a sensible starting point for many new aquariums. Heavily planted aquariums may eventually need a different lighting period, depending on the plants, light intensity and nutrient levels.

Leaving the light on for too long can encourage algae. Aquarium lighting should normally be switched off overnight so the fish have a period of darkness.

10. Test the Water

Before cycling begins, test your tap water and aquarium water for:

• Ammonia
• Nitrite
• Nitrate
• pH

Testing the source water gives you a useful reference point. Some tap water already contains measurable nitrate, and its pH and hardness will affect which fish are most suitable.

General hardness and carbonate hardness tests can provide additional information, particularly when choosing species with specific water requirements.

Clear water is not necessarily safe water. Harmful ammonia and nitrite cannot be identified simply by looking at the aquarium.

11. Cycle the Aquarium Before Adding Fish

A new filter does not immediately contain enough beneficial bacteria to process fish waste. The aquarium must therefore complete a nitrogen cycle before it is stocked.

During the cycle:

1. Ammonia is introduced into the empty aquarium using a controlled, fishless method.
2. Beneficial bacteria begin converting ammonia into nitrite.
3. A second group of bacteria converts nitrite into nitrate.
4. Water tests are used to monitor the process.
5. Fish are added only when the filter can process the ammonia source and both ammonia and nitrite return to 0mg/L within the required period.

A bacterial starter can help establish the filter, but it should not be treated as a substitute for water testing. A fishless cycle may take several weeks, and there is no fixed number of days that guarantees an aquarium is ready.

Do not add fish simply because the filter has been running for a week or because the water looks clear.

Our separate guide, How to Cycle a New Aquarium, explains the complete fishless cycling process.

12. Carry Out a Final Water Check

Before buying any fish, check:

• Ammonia is 0mg/L
• Nitrite is 0mg/L
• Nitrate is at an appropriate level
• The temperature is stable
• The pH is suitable for the chosen species
• The filter and heater are operating correctly

A partial water change may be required at the end of the cycle to reduce nitrate before fish are introduced. Treat all replacement tap water with water conditioner and match its temperature closely to the aquarium.

Bring a sample of aquarium water to Water World if you would like our team to test it and help confirm whether the aquarium is ready.

13. Add Fish Gradually

Do not fully stock a new aquarium in one visit.

Begin with a suitable number of compatible fish, taking account of whether the chosen species needs to live in a group. Adding fish gradually allows the biological filter to adjust to the increased amount of waste.

Before purchasing, check:

• The fish’s adult size
• Minimum aquarium dimensions
• Group-size requirements
• Temperament
• Water temperature
• Preferred pH and hardness
• Swimming level
• Compatibility with existing fish
• Diet and feeding needs

Avoid using simple stocking rules based only on the combined length of the fish. Body shape, behaviour, activity, waste production, territorial needs and adult size are all important.

Ongoing Care and Maintenance

Daily

• Check that all fish are present and behaving normally.
• Check the temperature.
• Make sure the filter and heater are operating.
• Look for signs of illness, injury or distress.
• Remove uneaten food where practical.
• Check that the aquarium has no leaks.

Weekly

• Test the water, especially in a newly established aquarium.
• Carry out a partial water change based on the test results and needs of the aquarium.
• Use a siphon to remove debris from accessible areas of substrate.
• Clean algae from the viewing panels if required.
• Check plant health.
• Inspect equipment and water flow.

A weekly partial water change of approximately 20–30% is suitable for many aquariums, but the correct amount depends on stocking, feeding, plant growth and water-test results.

Regularly

• Rinse reusable filter media gently in old aquarium water when the flow begins to reduce.
• Clean the filter impeller according to the manufacturer’s instructions.
• Trim live plants when needed.
• Inspect plugs, cables, pipes and seals.
• Replace consumable filter media only when necessary.

Never rinse biological filter media under untreated tap water. Chlorine and chloramine may damage the beneficial bacteria.

Avoid replacing all filter media at the same time. Many replacement schedules printed on filter packaging apply to particular chemical media and should not automatically be followed for mature biological media.

Common Problems and How to Avoid Them

Adding Fish Too Soon

A new aquarium may look ready long before its biological filter is mature. Adding fish before the cycle is complete can expose them to toxic ammonia and nitrite.

Prevent this by completing a fishless cycle and confirming the results with a reliable test kit.

Choosing an Aquarium That Is Too Small

Small aquariums can experience rapid changes in temperature and water chemistry. They may also be unsuitable for the adult size or social needs of the fish.

Research your preferred species first and choose an aquarium that provides enough long-term space.

Overstocking

Too many fish can lead to poor water quality, low oxygen levels, stress and aggression.

Stock gradually and consider the adult size, behaviour and waste production of every species.

Forgetting to Condition Tap Water

Untreated tap water can harm fish and filter bacteria.

Use a suitable water conditioner whenever tap water is added, including during small water changes and top-ups.

Washing the Filter Under the Tap

Untreated tap water may kill or damage the bacteria that make biological filtration possible.

Rinse mature filter sponges and biological media gently in water removed from the aquarium.

Replacing All the Filter Media

Replacing all the mature media at once can remove much of the biological filter and cause ammonia or nitrite to rise.

Replace media only when necessary and stagger any changes wherever possible.

Overfeeding

Uneaten food decomposes and adds extra waste to the aquarium.

Feed measured portions appropriate for the species and remove food that remains uneaten.

Leaving the Lights on Too Long

Long lighting periods and direct sunlight can encourage algae.

Use a timer and begin with a controlled lighting period of around six to eight hours.

Choosing Incompatible Fish

Fish may require different temperatures, water chemistry, group sizes or levels of water movement. Some species may also be territorial or predatory.

Research compatibility before purchasing and ask the Water World team if you are unsure.

Frequently Asked Questions

How long should a new aquarium run before fish are added?

There is no reliable answer based only on time. The aquarium is ready when testing shows that its biological filter can process the chosen ammonia dose and return both ammonia and nitrite to 0mg/L within the required period.

This often takes several weeks. Water-test results, rather than the number of days the aquarium has been running, should determine when fish can be introduced.

Can I add tap water directly to an aquarium?

Tap water should be treated with a suitable water conditioner before it comes into contact with fish or mature filter media. The replacement water should also be close to the aquarium’s temperature.

Do I need a filter?

Yes. A properly selected and maintained filter is essential for almost all freshwater fish aquariums. It removes debris, circulates the water and provides a home for the bacteria that process toxic waste.

Do coldwater fish need a filter?

Yes. Coldwater fish still produce ammonia and other waste. They require suitable filtration even if a heater is not necessary.

Does every freshwater aquarium need a heater?

No. A heater is required when the room temperature cannot keep the water reliably within the needs of the chosen species. Tropical species normally require one, while some temperate or coldwater species may not.

Always research the requirements of the individual fish.

Can I use bottled or distilled water?

Bottled, distilled and reverse-osmosis water do not automatically provide suitable aquarium water. Their mineral content and chemistry may be inappropriate or unstable.

Do not change the water source without understanding how it will affect pH, hardness and the fish. Ask our team for advice if your tap water is unsuitable for the species you want to keep.

Can I put fish in on the same day I set up the aquarium?

A conventional new aquarium should be cycled before fish are introduced. Adding fish immediately can expose them to ammonia and nitrite while the biological filter develops.

How many fish can I keep?

There is no single rule that works for every aquarium. Safe stocking depends on:

• Aquarium dimensions and water volume
• Adult fish size
• Species behaviour
• Group requirements
• Filtration
• Oxygenation
• Water chemistry
• Maintenance routine
• The amount of waste produced

Ask for a stocking plan based on the exact aquarium and species you are considering.

Why is my new aquarium cloudy?

Cloudiness may be caused by substrate dust or a temporary bacterial bloom. Do not add treatments without identifying the cause.

Test ammonia and nitrite, avoid overfeeding and allow the filter to run continuously. Ask for advice if the cloudiness persists or fish are already present.

Our Water World Tip

Plan the fish before you plan the aquarium.

Choosing your intended species first allows you to select the correct aquarium size, filter, heater, substrate and decorations. It is much easier to create the right environment from the beginning than to correct an unsuitable setup after fish have been introduced.

If you are unsure, bring your aquarium measurements and a sample of your tap water to Water World. Our team can help you create a suitable setup and stocking plan.