How to Read Aquarium Water Test Results Like a Pro

Testing your water is only useful if you can accurately interpret the results. Many beginners test faithfully but struggle to connect the numbers to actions. This guide from Gensou Aquascaping at 5 Everton Park teaches you how to read aquarium water test results with confidence and know exactly what each parameter means for your fish and plants.

Ammonia (NH₃/NH₄⁺)

Ideal reading: 0 ppm in any established tank.

Any detectable ammonia in a cycled tank signals a problem — overfeeding, a dead fish, filter failure or overstocking. At 0.25 ppm, fish become stressed. At 0.5 ppm and above, gill damage begins. At 1.0 ppm or higher, treat it as an emergency: do an immediate 50 per cent water change and dose Seachem Prime. During cycling, ammonia readings up to 4 ppm are expected and normal — just do not add fish until it drops to zero.

Nitrite (NO₂⁻)

Ideal reading: 0 ppm in any established tank.

Like ammonia, any nitrite in a cycled tank indicates a problem with biological filtration. Nitrite is toxic at even low concentrations. At 0.25 ppm, fish may show stress. At 0.5 ppm, perform a water change and add aquarium salt (1 g/L). During cycling, a nitrite spike to 2–5 ppm is normal and expected — it peaks after ammonia drops and then falls as the second group of bacteria establishes.

Nitrate (NO₃⁻)

Ideal reading: 5–20 ppm for planted tanks, below 40 ppm for fish-only tanks.

Nitrate is the end product of the nitrogen cycle and is relatively non-toxic at moderate levels. In planted tanks, nitrate is actually a nutrient — levels below 5 ppm can starve plants. Above 40 ppm, sensitive fish may show stress, and algae becomes more likely. If nitrate keeps climbing despite water changes, you are probably overstocking or overfeeding. Important tip: Shake API Nitrate Test Bottle #2 vigorously for 30 seconds and the test tube for 60 seconds — inadequate shaking is the number one cause of false zero readings.

pH

Ideal reading: Depends on your fish. Most tropical community fish thrive at 6.5–7.5. African cichlids need 7.8–8.6. Discus and Caridina shrimp prefer 5.5–6.5.

A stable pH matters more than a perfect number. Fish adapt to a range of pH values but suffer when pH swings rapidly. If your tap water is pH 7.5 and your fish prefer 7.0, do not chase the number — just keep it stable. Large pH swings happen when KH is low (below 3 dKH), making the water vulnerable to acid crashes. Test pH at the same time each day, as CO2 injection and photosynthesis cause natural daily fluctuations.

GH (General Hardness)

Ideal reading: 4–8 dGH for most tropical fish, 10–20 dGH for African cichlids and livebearers, 4–6 dGH for Caridina shrimp.

GH measures dissolved calcium and magnesium. It affects fish osmoregulation, shrimp moulting and plant nutrient availability. Singapore tap water is typically 2–4 dGH — soft enough for most soft-water species but may need boosting for livebearers or shrimp. Raise GH with remineralising salts; lower it by blending with RO water.

KH (Carbonate Hardness)

Ideal reading: 3–8 dKH for most setups, 1–2 dKH for planted tanks with CO2.

KH is your pH buffer. Higher KH resists pH changes; lower KH allows pH to move more freely. In CO2-injected tanks, lower KH lets CO2 lower the pH more effectively. But if KH drops to zero, your tank is at risk of a dangerous pH crash — the water can suddenly drop from 7.0 to 5.0 overnight. Always maintain at least 1 dKH in any tank.

How Often to Test

During cycling: test ammonia, nitrite and nitrate every two to three days. In a new tank (first three months): test weekly. In a mature, stable tank: test monthly or when you notice behavioural changes in fish. After any event — medication, new fish, power outage, major water change — test the next day. Keep a log of your results to spot trends over time.

Related Reading

• How to Read Aquarium Water Test Results: Colour Chart Decoded
• Green Water in Your Aquarium: Causes and How to Fix It
• Singapore Water Hardness Map: What Your Tap Water Means for Fish
• Aquarium Water Parameter Cheat Sheet: Ideal Ranges for Every Setup
• Aquarium Water Parameter Log Template: Track and Trend