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I have read this question, and this one, about goldfish/pond life suddenly dying. These situations are similar, but not identical to my own, and so I'm not sure whether this question is a duplicate.

We came back from a weeklong holiday yesterday, and had a look in our pond. We were happy to see that our two adult goldfish, and their four new babies were doing rather well. We pulled out quite a lot of pond weed, and topped up the pond with water from the hose. We also sprayed some of the surrounding plants with the hose, and some of the water may have run off into the pond.

We have woken up this morning to find all the fish dead, overnight. There were several tiny fish which we hadn't noticed (they look like recently hatched eggs) which are also dead - around 7 total in all.

What could have caused this? Reading around, it looks as though we may have had an issue with oxygen in the water - my intuition tells me that topping the pond up and pulling out (admittedly a lot of) pond weed shouldn't have caused a problem like that, but is that right?

It looks almost as if the fish were all electrocuted, but our pond also contains rather a lot of water snails, and they're all fine. We have in the past had several frogs residing in the pond, and there's no sign of them (either dead or alive).

The pond also has a lily in it, the flowers of which were pushed under water by the filling up of the pond last night. The pond is about 1 by 2 metres.

We also have two cats (it seems clear that the cats didn't do this, on the basis that all the tiny fish are also dead, and the bigger fish don't look damaged physically on the outside), and whilst I don't think that they drink from the pond, I'd like to confirm that we shouldn't panic and drain the pond or anything right now?

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    please take a water test and post the result PH-AMMONIA-NITRITE-NITRATE-CHLORINE,the chlorine test is the most important but i need the others too to make an answer.have you used any fertilizer or pesticides on the plants and what type of filter/pump and flow rate do you have,do not drain the pond yet wait until the cause is found for your problem.
    – trond hansen
    Jul 14, 2019 at 17:34
  • @trondhansen is right, get the readings. But bear in mind that big changes aren’t great for fish. I’d expect removing pond weed to result in more pond weed or a lot of algae, though, not fish death. Did you use a dechlorinator and do you know if your water supply contains chloramine? This seems the most likely cause.
    – Pam
    Jul 14, 2019 at 20:33
  • We have taken the decision not to take a water test - after further investigation we think that pulling out the pond weed disturbed a lot of silt/decomposed plant matter at the base of the pond, and this resulted in a lowering of the oxygen level in the water. When we observed the fish after removing the pond weed, they were darting around with their heads close to the surface, as if they were eating. We have now learned that this is a behaviour exhibited when oxygen levels are low in the water. This lends a little more credence to our theory about the oxygen levels.
    – Matt
    Jul 14, 2019 at 22:42
  • What do you mean" sprayed surrounding plants" ? What do you mean" topped up" ? . You can easily add 10% tap water any time ; I do it frequently in ponds and aquariums . I do not add more than 30 % tap water. Our municipal water does have chlorine type stuff in it because I made a bad mistake and left a hose running slowley into the pond and killed 5 large ( 22 ") koi,
    – blacksmith37
    Jul 15, 2019 at 0:41
  • When I clean leaves out of my pond, stirring up the bottom releases hydrogen sulfide, enough to smell bad . But it has never hurt the fish , so I doubt pulling "pond weeds" caused the problem. I see two possibilities , you added over 50% of the pond volume with city water or you sprayed something on the plants. Spraying plain city water on the margin plants would not hurt.
    – blacksmith37
    Jul 15, 2019 at 0:57

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