Succeeding with Your Freshwater Aquarium from A to Z: The Complete Guide
A successful freshwater aquarium comes down to one idea: it's not the water you maintain, it's the biological filter. Fish produce waste, bacteria convert it, and the whole job consists of letting these bacteria settle in before adding fish, then never rushing them. This guide follows the actual order of a setup: startup and nitrogen cycle, nitrites, filtration, volume and population, maintenance, then algae. Each step links to the corresponding detailed article.
3 to 6 weeksto complete the nitrogen cycle of a new tank (rough estimate)
0 mg/Lof nitrites expected in a balanced aquarium
20 Lof water minimum per goldfish
20 to 30%of water changed every week, depending on the tank
1. Understanding the nitrogen cycle, the key to everything
Food scraps, waste, dead leaves: everything decomposes into ammonia, toxic to fish. Bacteria, settled mainly in the filter media, convert it into nitrites, then a second group of bacteria converts the nitrites into nitrates, which are much less toxic. Nitrates are removed with water changes, and plants absorb some of them.
The point that many beginners ignore: the bacteria that manage nitrites settle in more slowly and tolerate disturbances less than those that manage ammonia. This is why the nitrite spike comes after the ammonia spike and often lasts longer. On a new aquarium, allow for a rough estimate of three to six weeks, sometimes more in cold or very soft water. For details: What are nitrites in an aquarium?.
2. Starting your aquarium, step by step
Cycling without fish is the safest method: we let the colonies form before any fish depends on them.
- Choose the location. A solid, stable, and level support (a full aquarium can no longer be moved), away from a radiator, near an outlet, sheltered from direct sunlight which promotes algae and green water.
- Install the decor and fill. Rinse sand, stones, roots, and decorations, put the substrate in place, then fill at room temperature by pouring the water onto a plate so as not to dig the bottom.
- Condition the water. A conditioner neutralizes the chlorine in tap water, which would destroy the filter bacteria.
- Start the filter and never turn it off. Bacteria need an oxygenated water flow day and night.
- Seed with bacteria and feed the colony. Add live nitrifying bacteria according to the manufacturer's dose, and provide a very small source of ammonia every day, a pinch of food for example.
- Test, then introduce the fish. The cycle is complete when ammonia and nitrites remain at zero and nitrates appear. The fish then arrive in small groups, with two to three weeks between introductions.
What we recommend for this startup phase:
Aquipond NitriBoost Aquarium 150 mlLiquid nitrifying bacteria to help set up the cycle. According to the product sheet: 10 ml for 50 L every 2 days for 2 weeks at startup.€19.95Bactocool Aquarium 100 mlLive bacteria to add at startup, after a filter cleaning or after a treatment.€12.99
Bacteria shorten the setup period, they do not replace tests. It is the result of the test, not the calendar, that says when the fish can arrive. Useful sections: bacteria for aquariums and water conditioners.
3. Nitrites: read the test and react in order
In a balanced aquarium, nitrites are undetectable. They cross the gills and prevent the blood from carrying oxygen: the fish suffocates in water that is nevertheless well oxygenated. Fish stuck to the surface, rapid breathing, loss of appetite: these are the first signs. See also Why do my fish stay at the top of the aquarium?.
| NO2 test reading | Interpretation | Reaction |
|---|---|---|
| 0 or undetectable | Functional biological filter | None |
| Traces, around 0.1 mg/L | Slightly overwhelmed filter or in cycling | Monitor, reduce food |
| 0.3 to 0.5 mg/L | Clear imbalance | Water change during the day |
| Above 0.5 to 1 mg/L | Toxic spike for most species | Emergency: complete protocol |
Rough estimates: sensitivity varies depending on species, temperature, and hardness.
Nitrite spike with distressed fish: change 30 to 50% of the water with conditioned water at the same temperature, stop feeding for 24 to 48 hours, oxygenate, and above all do not wash the filter. The complete timeline, hour by hour, is in Nitrites (NO2) in aquarium: the emergency protocol to lower the level in 48 hours.
When nitrites return from one week to the next, it is not the water that needs to be corrected but the capacity of the biological filter: incomplete cycle, filter too cleaned, population too large, KH too low, or undersized filter. The seven causes and the four-week plan: Nitrites returning in the aquarium: find the cause and re-establish a stable biological filter.
Two reflexes that avoid most bad surprises:
Sera Test NO2, drop testThe test we use to confirm a spike and monitor its decline morning and evening, finer than a strip.€11.35JBL Biotopol 500 mlWater conditioner that neutralizes tap chlorine: essential so that new water does not damage the filter bacteria.€16.82
Then, test nitrites every week, and three to five days after any change (new fish, filter cleaning, treatment). To choose your tests, see What water tests for a freshwater aquarium? and our analysis of your aquarium water section.
4. Filtration: the engine of the aquarium
A filter combines three functions: mechanical (the sponges retain waste), biological (the support for bacteria, sponge and ceramic, to be favored in volume) and chemical, occasional (activated carbon retains odors and color). To choose, two criteria: the volume of the tank and above all the population. For small fish, a filter that circulates about three times the volume of the tank per hour is enough; for goldfish or large cichlids, count rather five times. At equal flow, prefer the model that contains the most filter media. The details: What filtration flow for an aquarium?.
As soon as the population grows, we recommend an external filter. Two models we offer:
Oase BioMaster 350External filter for aquariums up to 350 L: Hel-X biological filtration, sponges and EasyClean pre-filtration chamber for maintenance without complete disassembly.€172.46JBL CristalProfi e702 greenlineExternal filter for aquariums from 60 to 200 L: a solid base as soon as a medium-sized tank welcomes goldfish or a large community.€144.15
The filters provided with some startup kits are barely sufficient for a light population. If nitrites return as soon as you add a few fish, switch to a larger filter, and keep the old one running for four to six weeks so that the new one is colonized. See also our external filters for aquarium section.
5. Choosing your volume and your population
The larger the volume, the more stable the parameters remain: one too many granules is diluted, an dosing error weighs less. In a nano-tank of less than 30 liters, everything goes faster. Some solid guidelines:
- Goldfish: 20 liters per goldfish, at minimum. That is 40 liters for a couple, and three goldfish maximum in 60 liters. A goldfish produces a lot of waste: plan a generous filter.
- Think about the adult size, not the size of the fish in the store.
- Measure pH and hardness (GH, KH) of your tap water before choosing the species: it is easier to choose fish adapted to your water than to modify the water for demanding species.
- In 60 liters, aim for about two species, spread across the three levels (bottom, mid-height, surface).
- Introduce the fish in small groups, at two or three week intervals.
To put into practice: How many fish in a 60L aquarium?, 10 ideal aquarium fish for beginners and Choosing an aquarium for a goldfish.
To start with a coherent set, we recommend the Superfish Start kits. A complete kit also makes a beautiful gift idea, and we have gathered 10 gift ideas for the owner of a freshwater aquarium.
Aquarium Start 70, black kit, 60 L, SuperfishComplete kit with LED lighting, internal filter and accessories. 60 liters: a base for a small tropical community, or for goldfish at a rate of 20 L per fish.€169.00Aquarium Start 100, black kit, 100 L, SuperfishComplete kit with LED lighting, internal filter and accessories. 100 liters: a beautiful margin for a couple of goldfish or a larger community.€229.95
Free delivery from 149 € (excluding bulky products). Find the entire range in our Superfish aquariums section.
6. Maintenance: a short, but regular routine
A stable aquarium requires few actions, provided they are regular: the challenge is to preserve the colonies of bacteria.
| When | What | How |
|---|---|---|
| Every week | Change 20 to 30% of the water | Conditioned water, same temperature; siphon the dirty areas without turning over all the bottom |
| Every week | Test nitrites | Drop test, recorded values |
| When the flow drops | Rinse the filter media | In a bucket of water taken from the aquarium, never from the tap; never all the media at once |
| Every month | Check pH and KH | More often if your water is soft |
| At each meal | Dose the food | An amount consumed in one to two minutes; what falls to the bottom is too much |
Two mistakes come back again and again: cleaning the filter on the day of the complete siphoning of the bottom, and changing 80% of the water because it smells bad. Better a moderate change every week, and look for the cause of the odor (see Aquarium that stinks, what to do?).
On the food side, one day of fasting per week does not bother most adult fish, and food adapted to the species limits waste. Our aquarium fish food section allows you to choose according to your species; for quantities, see for example What quantity of food to give a betta fish?. The pace of water changes is detailed in How to change the water of an aquarium?.
7. Algae: rebalance rather than fight
A little green algae on the glass is normal: a swipe of a scraper or magnet during the weekly maintenance is enough. When they settle everywhere, it is the sign of an imbalance, and we correct it by acting on the causes rather than looking for a radical solution. Five levers, to be activated together:
- Nutrients: measured food, siphoning of waste, regular water changes.
- Light: no direct sun on the tank, reasonable lighting duration.
- Bacteria: a stable biological filter consumes the organic matter that feeds the algae.
- Oxygen: surface agitation or air pump, see How to know if there is enough oxygen in the aquarium?.
- Plants: fast-growing plants absorb part of the ammonia and nutrients, and take the place of algae.
“Black algae” often refers to two different problems: beard algae and cyanobacteria. A pH above 7.5 and a high KH favor them. There is no instant solution: we rebalance the water step by step. Our most visited article on the subject: Black algae in an aquarium: understand, limit and prevent.
For water that turns green, see Why is the water of my aquarium green?; for a blue-green veil on the bottom and decor, How to reduce cyanobacteria in an aquarium?
8. Checklist before adding the first fish
- The aquarium rests on a solid, stable and level support.
- The water is conditioned, and the filter has been running day and night for several weeks.
- Nitrifying bacteria have been added, and the colony is fed.
- Ammonia and nitrites are at zero on several spaced tests, nitrates have appeared.
- The population is adapted to the volume (20 liters per goldfish) and will be introduced in small groups.
- A weekly water change is planned, as well as a nitrite test every week.
Frequently asked questions
How long do you have to wait before introducing the fish?
Roughly three to six weeks in temperate water, sometimes longer in cold or very soft water. The cycle is complete when ammonia and nitrites remain at zero over several tests and nitrates appear.
How many liters are needed per goldfish?
Allow at least 20 liters of water per goldfish, or 40 liters for a pair. In a 60-liter aquarium, a maximum of three goldfish.
What should I do if the nitrite test is high?
Change 30 to 50% of the water with conditioned water at the same temperature, stop feeding for 24 to 48 hours, increase aeration, and do not clean the filter. Retest morning and evening.
Should I turn off the filter at night?
No. The bacteria in the filter need a constant flow of oxygenated water. Turning off a filter for several hours suffocates part of the colonies.
How often should I change the water in an aquarium?
Once a week, generally 20 to 30% of the volume with conditioned water at the same temperature. Consistency is key.
How to limit algae without harsh products?
By rebalancing the aquarium: fewer nutrients (measured food, regular water changes), no direct sunlight, a stable biological filter, good aeration, and fast-growing plants.
Learn more
- Nitrites (NO2) in an aquarium: the emergency protocol to lower the level in 48 hours
- Nitrites returning in the aquarium: find the cause and restore a stable biological filter
- Black algae in an aquarium: understand, limit, and prevent
- Why do my fish stay at the surface of the aquarium?
- How to lower the pH of an aquarium?
- 10 ideal aquarium fish for beginners
- Stinky aquarium, what to do?
- How many fish in a 60L aquarium?
- How to test aquarium water?
- How to properly maintain your aquarium?
Unsure about your results? Tell us the volume of your aquarium, the date it was filled, the filter model, the population, and your latest tests (nitrites, pH, KH): the Foudebassin team will help you find the cause.
Contact the teamView water testsExpert advice from the Foudebassin team since 1998. Last updated: November 1, 2026.
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