It sounds like your pond has high levels of nitrate
How to be sure
Your test results show high nitrate levels and your pond might also have quite a bit of algae in it. The fish may have rapid gill movement and be gasping at the water surface.
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.