Water
Nitrite: watch the filter’s second step
Nitrite can appear while a filter is establishing or after its capacity is disrupted.
Why it matters
A pond can have an improving ammonia result while nitrite is still a problem. Test both. Do not treat clear water or a single good result as evidence that the filter is ready for more fish.
What to check
Review feeding, recent additions, oxygen supply and whether biological media has been replaced or cleaned aggressively. Confirm that water is passing through the filter as intended.
How to respond
Stop adding fish, assess the existing fish and obtain advice on reducing the waste load and using suitable conditioned water changes. Keep a record so you can see whether the result is improving or recurring.
Do not guess a salt dose
Species, existing salt, plants, other treatments and the kit’s units all matter. A generic internet dose is not a substitute for pond-specific guidance. Tell a specialist exactly what has already been added.
Keep the sequence
Record the last few nitrite readings and the dates of new fish, filter work or water changes. Include the test kit and units. A sequence gives more context than describing the colour as simply pink or purple.
Sources & further reading
Original WakeyKoi wording, informed by the references below. Linked clinical resources explain the evidence and their scope; they have not reviewed or endorsed this website.
Related guides
Educational guidance cannot diagnose an individual fish. Breathing distress, severe injury or unexplained losses need prompt professional assessment.