It sounds like your pond has high levels of nitrite
How to be sure
Your test results show high Nitrite levels and your fish aren’t happy. They may appear pale or dark and shimmy, scratch, jump and flick against objects in the pond. Rapid gill movement, brown gills and breathing at the water surface are also signs of your fish’s discomfort.
Step 1 – Treat your pond
BE AWARE! – Treatments can sometimes reduce oxygen levels in the water. If your fish begin gasping at the surface you can add an air pump to increase oxygenation.
Step 2 – Support your fish
Blagdon water and algae treatments can safely be used with Blagdon fish medications.
Only use one fish medication at a time.