Question
Hi
I live in Australia and we have a large pond, about a thousand litres, with some large goldfish, that have lived happily for years. In the last couple of weeks, pond went blackish, smelt very chemically, fish gasping on surface.
We've tried everything! tested, replaced, filtered aerated, and each time after a couple of days, the smell is back and the fish are gasping on the surface again! I feel like a serial killer!! Thanks

Answer
I'm sorry that you've lost fish, and something is obviously really wrong.  How is the pond lined?  Is it a PVC liner, dirt pond, or something else?  Can you describe the smell?  Is it like rotten eggs (hydrogen sulfide), feces (methane), dead fish (dead animals), or something else?  What did you test?  What were the results?  What kind of aeration and filtration do you have?  Gasping at the surface normally indicates low oxygen levels.  Since you aerated, they must be really low from something wrong.  The smell may indicate a turnover of rotting animals and/or plants.  What is on the bottom of the pond; is there debris?  If the pond is not lined (just dirt), then a literal gas leach could be effect with toxic gases coming up and coloring the water and smelling.  If it's lined and clear of debris (rotting plants or animals), then I'm not sure why that would happen.  Have you done water changes?  If not, do that right away to reduce the black stuff and smells.  You need to find the source of the problem.  Could fecal waste have gotten into the pond?  Has there been a lot of rain?  I'm trying to help figure this out but sorry if I wasn't of much help!

Robyn
fishpondinfo.com