It sounds like your pond has sludge
How to be sure
You have a huge amount of dirty, sludgey, waste building up in your pond. It clouds when disturbed and released nutrients that could produce algae problems
Step 1 – Treat your pond
TIP: Use an ongoing preventative dose of Clean Pond Pods to maintain a clear, healthy pond once the sludge has gone. Brilliant Pond contains three active ingredients that eliminate sludge, blanketweed and reduce filter maintenance.
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.