How to Set Up a Fish Tank: Equipment, the Nitrogen Cycle and Your First Fish
Most new-tank fish losses come from adding fish too soon. Here is the equipment you need and a step-by-step plan for cycling a freshwater aquarium.
To set up a fish tank, choose a tank that is as large as your space and budget allow, add a filter, heater (for tropical fish), substrate and decor, fill it with dechlorinated water, and then cycle the tank before adding fish. Cycling lets beneficial bacteria grow in the filter so they can turn toxic fish waste into less harmful compounds. It usually takes several weeks, and the RSPCA advises waiting at least two weeks after setup before adding any fish.
This guide focuses on a basic freshwater aquarium. Marine (saltwater) tanks are more complex and are not covered here.
What you need
| Item | Why it matters | What to look for |
|---|---|---|
| Tank | Larger volumes stay stable longer | Sized for the adult fish you plan to keep; sturdy lid |
| Stand | Water is very heavy | Rated for the full weight; level and away from direct sun |
| Filter | Moves water and houses beneficial bacteria | Rated for your tank volume; gentle flow for slow swimmers |
| Heater and thermometer | Tropical fish need stable warmth | Adjustable heater with thermostat; separate thermometer to check |
| Water conditioner | Removes chlorine and chloramine from tap water | A dechlorinator that treats both |
| Test kit | Lets you track the cycle and water quality | Tests for ammonia, nitrite, nitrate and pH |
| Substrate and decor | Shelter, hiding places, surface for bacteria | Aquarium-safe gravel or sand; smooth decor; live plants if you like |
| Light | Day-night rhythm and plant growth | On a timer, typically for part of the day only |
A larger tank is usually easier for beginners than a tiny one, because waste and temperature changes are diluted in more water.
The nitrogen cycle in plain English
Fish constantly release ammonia through their gills and waste, and uneaten food breaks down into ammonia too. Ammonia is toxic to fish. In an established tank, two groups of bacteria living mainly in the filter handle it:
- The first group converts ammonia into nitrite, which is also toxic.
- The second group converts nitrite into nitrate, which is far less harmful at low levels.
- You remove nitrate with regular partial water changes.
A brand-new tank has almost none of these bacteria. If you add a full load of fish straight away, ammonia and nitrite can build up and harm or kill them. Fishkeepers call this “new tank syndrome.”
Step by step: setting up and cycling
- Place the tank. Pick a level, strong surface near a power outlet, away from radiators, drafts and direct sunlight, which can cause temperature swings and algae.
- Rinse substrate and decor in plain water (no soap), then add them.
- Fill with water treated with conditioner. Pour onto a plate or bag to avoid disturbing the substrate.
- Install and switch on the filter and heater. Set the heater for the species you plan to keep and confirm with a thermometer.
- Start the cycle. The RSPCA recommends fishless cycling: adding a small source of ammonia, such as a pinch of fish food or an aquarium ammonia product used as directed, so bacteria can grow before any fish arrive. Some products add live bacteria to help speed things up.
- Test regularly. Expect ammonia to rise, then nitrite to rise as ammonia falls, then nitrite to fall as nitrate appears.
- Wait for zeros. The tank is generally considered cycled when ammonia and nitrite both read zero and nitrate is present. This often takes several weeks; don’t rush it.
- Do a partial water change before adding fish, to bring nitrate down.
The RSPCA’s fish welfare advice has more on preparing a tank and choosing a responsible source for fish.
Choosing and adding your first fish
Research each species’ adult size, temperature range, water needs and temperament before you buy, and make sure they are compatible with each other. Some fish need to live in groups; others, like male bettas, must not share a tank with their own kind. Our betta fish care guide covers one popular single-fish option.
- Start small. Add a few hardy fish first, then wait and test before adding more, so bacteria can catch up with the extra waste.
- Buy from a reputable source. The RSPCA suggests checking that shop tanks are clean, with no dead fish, and that fish swim easily without split fins, white spots or sores.
- Acclimate gently. Float the sealed bag to match temperature, then follow the retailer’s guidance for introducing the fish.
- Feed lightly. Offer only what the fish eat quickly; leftover food pollutes the water.
Ongoing maintenance
| How often | Task |
|---|---|
| Daily | Feed, check temperature, count fish and look for unusual behavior |
| Weekly | Partial water change with conditioned water at a similar temperature; test water |
| Every few weeks | Rinse filter media in removed tank water, never under the tap, so bacteria survive |
| As needed | Vacuum gravel, trim plants, clean glass |
Warning signs to act on
Test the water first if you see fish gasping at the surface, clamped fins, sudden lethargy, loss of appetite, white spots, cloudy eyes, sores or rapid deaths. Poor water quality is a common cause. If problems continue, ask a veterinarian who treats fish or an experienced aquatics specialist for advice; our guide to choosing a veterinarian explains how to find one.
Bottom line
A healthy aquarium is mostly about patience. Set up the equipment, cycle the tank until ammonia and nitrite read zero, add a few compatible fish at a time and keep up with weekly water changes. Do that and you avoid the most common cause of early losses.
FAQ
How long should a new tank run before adding fish?
The RSPCA advises at least two weeks, but full cycling often takes several weeks. Use a test kit and wait until ammonia and nitrite read zero.
Can I use tap water in an aquarium?
Usually yes, once treated with a water conditioner that removes chlorine and chloramine. Check your water’s pH and hardness for the species you want.
Should I clean the filter under the tap?
No. Chlorinated tap water can kill the beneficial bacteria. Rinse filter media gently in water removed from the tank during a water change.
What size tank is best for beginners?
Larger tanks are generally more forgiving because water conditions change more slowly. Choose the biggest tank that suits your space and the adult size of your fish.