For over 8 years I have had a pond in the corner of my home. It is of a triangular shape, about 10' x12' x15' (3 x 3.6 x 4.5 m) by two feet (60 cm) deep. For years I have had two koi, random other pond fish and some carnival won goldfish. The last three years or so, frogs have found a home there, too. I have a waterfall and a couple plants.

I've never had any problems and I do very minimal maintenance with no issues.

About a week ago, a fish or two died. For the next couple days, I lost two to three a day. On the third day, I had water tested and it was fine. Next day, more dead fish. I drained 2/3 of the water and refilled, also added stress coat/chlorine reducer from pet store. Next day, more dead. One koi was about seven inches (18 cm) and he held out long, but finally died, leaving just the one koi left. Two days later, that 12 inch (30 cm) koi died.

When no fish were left, suddenly the water got extremely clear. I saw nothing wrong with the fish, except they showed signs of a lack of oxygen. They would float like dead till you attempt to net them, then they would swim. No spots or visible redness was to be seen around gills. Water has been pumped into a fountain the same all year, so why would O2 levels suddenly drop within a week enough to kill all the fish? Water was not warm.

  • 1
    Did you have the water tested for chemicals, or for micro-organisms? If O2-levels drop without excessive heat, with the waterfall running and the plants all happy, all that is left is, really, micro-organisms "stealing" the oxygen....
    – Layna
    Jul 9, 2015 at 6:51
  • Ohh, another question: is this an indoor-pond? and if yes, did you have any excessive scum on the ground? (I have not yet found an outdoor-pond without it...)
    – Layna
    Jul 9, 2015 at 7:04
  • We experienced a similar issue... Is there any acidic leaf litter that may have effected the pond? How is other life like frogs/newts? Are the plants in your pond also uneffected? Our fish looked completely healthy when they all died also, but plants and newts survived.
    – Aravona
    Jul 9, 2015 at 13:41
  • I had the water tested and it was fine. Plants are all fine and sitting here now the frogs are croaking away fine. The pond is outdoors and I have not done a thing to the pond or surrounding areas. No chemicals or granular lawn treatments. Water and fish were fine a week ago. Water has its usual green tint nothing out of the ordinary, no excessive algae. Then fish die and water is now crystal clear. Like so clear I can see every little root to the plants at the bottom. It's like the first day I put water in it. That's what gets me, why did it get so crystal clear now.
    – Dustin
    Jul 9, 2015 at 17:06
  • 2
    Does your city spray for mosquitoes or other pests?
    – Spyral
    Jul 9, 2015 at 21:47

1 Answer 1


Based on the info in your question and comments: water tested "fine", plants are still alive, and death over time (not all in one day) suggests fish illness. Since you didn't mention any signs of sores, gasping for air, or swollen eyes or abdomen, I would assume you got a parasite. Your water will test fine and your plants will live. Doing a water change would've only helped if a treatment had begun. Parasitic infections (unless it's ich) can be a little harder to spot, since SYMPTOMS are less noticeable than bacterial infections, swim bladder, dropsy or pop-eye. ALOT of fish In pods hide in live plants, you may have never noticed if a few of your fish were hanging out at the bottom, rubbing up against plants or rocks, have swollen gills, or behave erratically at random (aka flashing-quick spurts of jumping or quick swimming or circular swimming).... These are all signs of a parasitic infection, that ESPECIALLY in ponds with or without live plants, is difficult to spot. Parasites are very common in outdoor ponds, and simply a windy day, where debris is blown from further away can cause an infestation.

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