Water Testing Explained
You can't see ammonia, nitrite or nitrate — but they decide whether your fish live or die. Here's what each one means, the safe range, and exactly what to do when it's off.
0 ppm
Safe ammonia
0 ppm
Safe nitrite
<40 ppm
Safe nitrate
Weekly
Test frequency
Ammonia
Safe
0 ppm
The most toxic parameter. Produced by fish waste and rotting food. Even small amounts burn fish gills and can kill quickly.
If it's high: Do a 30% water change immediately, stop feeding for a day, and make sure the tank has cycled (good bacteria built up).
Nitrite
Safe
0 ppm
The middle stage of the cycle. Bacteria convert ammonia into nitrite — still toxic and stops fish blood carrying oxygen.
If it's high: Big water change and add live bacteria. It means your tank is mid-cycle; keep testing until it reads zero.
Nitrate
Safe
Under 20–40 ppm
The end product of the cycle — much less harmful, but builds up over time and fuels algae.
If it's high: Regular 20–25% weekly water changes remove it. Live plants also absorb nitrate as fertiliser.
pH
Safe
6.5–7.5 (stable is best)
How acidic or alkaline the water is. Most community fish adapt to any stable pH in this range — chasing a 'perfect' number causes more harm than good.
If it's high: Don't chase a number. Keep pH stable. Only adjust if your specific species truly needs it, and do it gradually.
When to Test
- In a new tank: every few days until ammonia and nitrite both read 0 for two tests in a row (the cycle is complete).
- In a mature tank: once a week, or whenever fish look off (hiding, gasping, clamped fins).
- After adding new fish: test daily for a week — the bioload just increased.
- After any water problem: test the next day to confirm the change worked.
💡 Pro tip: Test strips are quick and fine for most beginners. Liquid test kits are more accurate and cheaper per test if you stay in the hobby. Either way — the only bad test is the one you don't do.
More than just fish
My First Pet helps with every first pet — explore our other beginner guides:
Recommended on Amazon
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Water Test Kit
Test ammonia, nitrite, nitrate and pH to keep your fish safe.
Check price on Amazon
Water Conditioner
Removes chlorine and heavy metals — essential for every water change.
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Live Bacteria Starter
Bottle of good bacteria to speed up tank cycling — use when setting up.
Check price on Amazon
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