Water testing for fish tanks: how to read ammonia, nitrite, nitrate (and what to do next)

Quick answer

Test ammonia, nitrite, and nitrate with a liquid kit. In a cycled tank, ammonia should be 0, nitrite should be 0, and nitrate should stay low (many community tanks aim for under 20 to 40 ppm). If ammonia or nitrite is above 0, do a partial water change, stop overfeeding, and check your filter and stocking.

The three tests that matter most

Ammonia (NH3/NH4+)

• Goal: 0
• Why it matters: it burns gills and stresses fish quickly.

If you see ammonia above 0:

• Do a 25% to 50% water change.
• Reduce feeding for 24 to 48 hours.
• Confirm your filter is running and media is not replaced all at once.

Nitrite (NO2-)

• Goal: 0
• Why it matters: it reduces oxygen transport in fish.

If you see nitrite above 0:

• Do a 25% to 50% water change.
• Add extra aeration and surface agitation.
• Avoid adding new fish until it returns to 0.

Nitrate (NO3-)

• Goal: low and steady
• Why it matters: high nitrate is a long-term stressor and often fuels algae.

If nitrate is high:

• Increase water change size or frequency.
• Review feeding amounts.
• Add easy live plants if you want a natural nitrate sink.

A simple testing schedule

• New tank or after changes: 2 to 3 times per week
• Stable tank: weekly, or every other week if you know your tank is stable
• Anytime fish act "off": test immediately

Consistency beats perfect schedules.

Quick interpretation table

• Ammonia 0, nitrite 0, nitrate rising slowly: normal
• Ammonia above 0: tank not fully cycled or overloaded
• Nitrite above 0: mid-cycle or filter bacteria disrupted
• Nitrate very high: maintenance routine needs adjustment

Common reasons numbers suddenly look bad

• Overfeeding (most common)
• Adding too many fish too fast
• Cleaning filter media in tap water
• Replacing all media at once
• Missing water changes for a few weeks

FAQ

Do I need to test pH too?

pH matters, but stability matters more. If your fish are suited to your tap water and pH is stable, do not chase perfect numbers with chemicals.

Are test strips okay?

They are better than nothing, but liquid kits are usually more accurate for ammonia and nitrite.

Why do my results look different after a water change?

Fresh tap water can shift readings slightly. Test again later the same day after the tank mixes.

Next step

Want a routine-based recommendation for fish that match how much maintenance you are willing to do?