Ammonia Spike in an Established Tank

In a tank that's been running a while, a sudden ammonia or nitrite spike usually means overstocking, overfeeding, a dead fish or plant matter decaying somewhere, or filter media that lost its bacteria (often from rinsing it in tap water). Find and remove the source, then support the bacteria colony while it recovers.

Why this happens

• Overstocking or a recent addition of several new fish at once.
• Overfeeding, or a dead fish or plant matter decaying somewhere unnoticed.
• Filter media that lost its bacteria colony, often from rinsing it in tap water (chlorine kills the bacteria) or replacing too much media at once.

What to do next

1. Find and remove the source — check for a hidden dead fish, decaying plant matter, or recent overfeeding.
2. Do partial water changes to bring ammonia down to a safe level while the bacteria colony recovers.
3. Add a beneficial bacteria starter product to help the colony rebuild faster.
4. Hold off on adding new fish or cleaning the filter aggressively until levels stabilize.

Frequently asked questions