Fish Tank Water Quality in a New Aquarium

Most fish losses in a brand-new aquarium happen quietly, in clear water, days or weeks after setup. The cause is usually the same: an immature biofilter that can’t yet process ammonia from fish waste, so invisible toxins build up faster than you can spot them.

Below is a practical, Australian-focused guide to water quality in a new tank: what “cycled” really means, which parameters matter first, how often to test, and what to do when the numbers (or the fish) start to look wrong—without overcorrecting and making the water swing.

Why water quality matters most in the first six weeks

A new aquarium can look stable long before it behaves that way. Filters and gravel are clean, the water is sparkling, and then—often around the first fortnight—ammonia or nitrite rises as the tank’s bacterial community struggles to keep up. This is the classic “new tank” period where small mistakes (overfeeding, too many fish, over-cleaning the filter) can quickly turn into a toxicity event.^{1, 2}

Understanding the nitrogen cycle (what “cycled” means)

Fish waste, uneaten food, and decaying plant matter break down into ammonia. Ammonia is highly toxic to fish at very low concentrations, and it becomes more toxic as pH and temperature rise.²

In a functioning aquarium, a biofilter hosts nitrifying bacteria that convert:

• Ammonia → Nitrite (still toxic)
• Nitrite → Nitrate (generally far less toxic; managed with water changes and plants)

A tank is usually considered “cycled” when ammonia and nitrite stay at zero while nitrate is being produced—often taking around 4–6 weeks in a typical new freshwater setup.²

The water parameters that matter (and what you’re aiming for)

In a new aquarium, prioritise the toxins first, then the comfort settings.

Ammonia, nitrite, nitrate

As a general guide for many tropical community tanks, the RSPCA notes these targets:

• Ammonia: < 0.1 ppm
• Nitrite: < 0.2 ppm
• Nitrate: < 50 ppm

Exact tolerances vary by species, but in a new tank, treat any detectable ammonia or nitrite as a prompt to act.¹

pH and temperature

pH is not a “one perfect number” across fish species, and stability matters as much as the reading itself. A commonly suitable range for many tropical fish is roughly pH 6.5–7.5, with a steady temperature appropriate to the species (many tropical fish sit around the mid-20s °C).¹

Be cautious with chasing pH using chemicals. Rapid swings can stress fish, and higher pH can increase ammonia toxicity, so it’s usually safer to focus on keeping ammonia and nitrite at zero and avoiding sudden changes.²

How often to test in a new aquarium

During startup, test often enough to catch the “quiet” spikes.

• First six weeks: test ammonia and nitrite every 2–3 days
• After the tank is cycled: test every 1–2 weeks, or after any change (new fish, filter maintenance, medication)

This schedule aligns with RSPCA guidance for new aquariums while the nitrogen cycle is establishing.^{1, 2}

Filtration: choose for biology, not just “clear water”

Aquarium filtration has three jobs: mechanical (traps debris), chemical (removes some dissolved compounds), and biological (houses the microbes that detoxify ammonia and nitrite). Biological filtration is the critical one in a new tank, and it depends on oxygenated water flowing through filter media with lots of surface area.²

Aeration matters here too. If your filter doesn’t disturb the surface much, add an airstone to keep oxygen levels up for fish and for the biofilter itself.³

Filter cleaning: the common mistake

Don’t “sterilise” the filter. Rinse sponges/media gently in a bucket of old tank water, not under the tap, because chlorine/chloramine can kill the very bacteria you’re trying to grow.²

Water changes that actually help (without crashing the cycle)

Partial water changes dilute toxins and export nitrate. In the early weeks—especially if ammonia or nitrite is rising—more frequent small-to-moderate changes are often safer than one dramatic reset, because they reduce toxins while keeping conditions steady for the developing biofilter.²

A sensible routine once things are stable

For many freshwater tanks, a common maintenance pattern is a 10–25% weekly water change, paired with a light gravel clean to remove waste.²

Dechlorination: chlorine and chloramine are a real risk

In Australia, some reticulated water supplies use chloramines (including monochloramine) as a disinfectant. Chloramines are harmful to aquarium animals, so new tap water should be treated with a suitable water conditioner before it goes into the tank.^{4, 5}

Conditioners vary in what they neutralise. If your area uses chloramine, make sure the product explicitly treats chloramine, not only chlorine.^{2, 5}

Preventing algae: keep the system lean and steady

Algae thrives on excess light and excess nutrients. In new tanks, nutrient swings are common—especially from overfeeding and from the first months of the nitrogen cycle.

• Keep lighting consistent and avoid very long photoperiods while the tank settles.
• Feed lightly (uneaten food becomes ammonia).
• Maintain nitrate control with routine partial water changes.²
• Don’t over-clean the filter or substrate all at once; keep changes gentle and regular.²

Troubleshooting common water quality problems

If ammonia or nitrite is detectable

Assume the tank is under-filtered for its current bioload (or the biofilter has been disrupted). Act early.

• Do a partial water change (often 10–25%; repeat as needed) to dilute toxins.²
• Reduce feeding for a day or two; remove uneaten food promptly.
• Check for obvious causes: a dead fish/snail, rotting plant mass, clogged filter, or an overstocked tank.
• Make sure the filter media is not being rinsed in tap water.²

If fish are gasping at the surface

Surface gasping can occur with poor oxygenation, high ammonia/nitrite, or other irritants. Increase surface agitation/aeration immediately and test ammonia and nitrite as your first check.³

If nitrate keeps climbing

Nitrate is usually managed with routine partial water changes and sensible stocking/feeding. If it climbs despite maintenance, it’s a sign the tank is carrying more waste than you’re exporting—often from heavy feeding, too many fish, insufficient plant uptake, or missed gravel cleaning.²

A note on “just add aquarium salt”

Salt is not a universal fix for water quality, and many freshwater fish and plants don’t appreciate routine salting. In some situations, chloride (from salt) can reduce nitrite uptake and may be used as a short-term measure under informed guidance, but it does not remove ammonia, nitrite, or nitrate from the water—the biofilter and water changes do that work.⁶

Final thoughts

A new aquarium is a small, enclosed ecosystem, still assembling itself. Keep the water steady, keep toxins at zero, and let the biofilter mature at its own pace. With regular testing in the first six weeks, gentle water changes, and careful filter handling, most “new tank” problems can be spotted early—while they’re still just numbers on a test kit.^{1, 2}

References

1. RSPCA Australia Knowledgebase: How should I care for my tropical fish?
2. RSPCA Australia Knowledgebase: Why is water quality important when setting up a fish aquarium?
3. RSPCA Australia Knowledgebase: How should I care for my goldfish?
4. NHMRC: Australian Drinking Water Guidelines – Chloramines (Part 4: Disinfection)
5. NHMRC: Australian Drinking Water Guidelines – Monochloramine
6. Petco: Using salt in freshwater aquariums (benefits and safe use)
7. Seachem: Prime (water conditioner) – overview and claims
8. Seachem Support: Prime dosing instructions
9. Seachem Support: Prime product description (chlorine/chloramine and ammonia binding)