Starting a freshwater aquarium: the first six weeks

Updated on August 25, 2026 · ~6 min read

For the first six weeks of a new tank you do not add fish: you grow the bacteria that will process their waste. Set up, run the filter, test ammonia and nitrite every 2-3 days, and add the first animals only once both hold at 0 ppm and nitrate has appeared.

A first aquarium nearly always goes wrong the same way: it gets set up on Saturday and stocked on Sunday. The tank looks ready — the water is clear, the filter is bubbling, the heater is warm — but biologically it is still empty, and the fish pay for it over the following fortnight.

This is the sequence that avoids that. The weeks are indicative: the tank follows its own biology, and the only thing that really decides when you move on is the test readings.

Week 0: before you add water

Three decisions belong here, because later they cost far more.

Volume. Bigger is more stable: in 200 L (53 gal) a mistake dilutes, in 20 L (5 gal) you pay for it immediately. A first tank is best not smaller than 60 L (16 gal), which is also the sensible minimum for most community fish.

Who is going to live in it. This feels premature and instead it decides everything else: water hardness, temperature, substrate, how heavily planted. A school of neon tetras and a group of Malawi cichlids want two different tanks — not two decorating styles, two different waters.

The water you actually have at home. Test your tap water's GH and KH before choosing livestock: it is far easier to pick fish that suit your water than to correct your water for years. For reference, a tropical community sits well around 6-12 dGH, soft-water fish like discus and neons at 2-6 dGH, African rift lake cichlids at 8-15 dGH.

Week 1: setup and starting the filter

Add substrate, arrange rocks and wood, fill with water treated with a dechlorinator (tap chlorine and chloramine kill the bacteria you need to grow), switch on filter and heater and bring the temperature to 25-27 °C (77-81 °F) — which is also the range where nitrifying bacteria grow best.

If you want plants, add them now: they take up ammonia and nitrate, and a planted tank starts with an advantage. Fast growers are the most useful at this stage, even if you remove them later.

Lighting, by contrast, stays short: 6 hours a day is plenty for plants and gives nothing to algae, which colonise an unbalanced tank first.

What not to do in week 1: add fish; add "tank starter" products that are neither bacteria nor dechlorinator; change water.

Weeks 1-3: getting the cycle going

Bacteria need ammonia to grow, and an empty tank has none. You have to provide it. The three routes are fishless cycling with dosed ammonia (the cleanest, because no animal spends weeks in toxic water), a few hardy fish, or a pinch of flake food every couple of days, which decays into the same thing.

From here on, test ammonia and nitrite every 2-3 days. Ammonia will climb at first: that is normal and it is proof the cycle started. In a fishless tank you do not intervene — lowering ammonia starves the bacteria and lengthens the cycle. If fish are present the reasoning flips: partial water changes to keep ammonia as low as possible, minimal food, and their safety ahead of the cycle's speed.

Weeks 2-5: the nitrite spike

At some point ammonia starts to fall and nitrite appears. This is the halfway mark and the longest phase: nitrite can stay detectable for two or three weeks.

Nitrite is nearly as toxic as ammonia — it prevents blood from carrying oxygen — so there is only one rule here: no new livestock until it returns to 0 ppm. There is nothing to do but wait and keep testing. Bottled bacteria may shorten the wait, sometimes a lot, sometimes not at all.

This is also when the first diatoms show up, the brown film on glass and leaves. They are normal in a young tank, they feed on the silicates in new water, and they disappear on their own when that runs out. Not a problem to solve — a stage to pass.

Weeks 4-8: confirmation and first fish

Once ammonia and nitrite both read 0 ppm, one check remains: nitrate. If it has appeared (above roughly 5 ppm) the chain is complete and the tank is working. Three zeros instead mean the cycle has not started — the same reading as a freshly filled tank, and telling the two apart requires knowing whether ammonia and nitrite have already been seen.

With the cycle confirmed: a water change to bring nitrate down, then the first animals. A few at a time, roughly ten days apart. Every addition adds load and the bacteria must multiply to match: stocking the whole tank in one afternoon is the classic way to restart ammonia in a tank that looked fine.

What to test, and how often

Stage	What	How often
Weeks 1-5	ammonia, nitrite	every 2-3 days
Confirming the cycle	nitrate	once, when NH₃ and NO₂ are at 0
Always	temperature	daily, a glance at the thermometer
Start, then as needed	pH, GH, KH	once in week 1, then monthly
Established tank	nitrate	before every water change

Freshwater reference values: ammonia and nitrite always 0 ppm; nitrate under 25 ppm (in a planted tank 10-25 ppm is fine — plants need it); community pH 6.5-7.5; KH 3-8 dKH; temperature 24-27 °C (75-81 °F).

The five mistakes that cost the most

1. Adding fish too early. By a wide margin the biggest killer of aquarium fish.
2. Not treating tap water. Chlorine does exactly its job: it disinfects, which means it kills the bacteria you are trying to grow.
3. Rinsing filter media under the tap. Most of the colony lives in those sponges. Rinse in tank water, and not all of them at once.
4. Overfeeding. Anything the fish do not finish in two minutes becomes ammonia. A fish can skip a day; it will not survive a month of loaded water.
5. Chasing pH with products. Fighting for a decimal with "pH down" produces swings that stress fish far more than a slightly off value. Work through KH, CO₂ and water changes, or choose fish that suit the water you have.

How Gurglee does it

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