5 of my 18 fish have died in the last 9 or 10 days; what's wrong/what can I do?

May 26, 2010
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I have a small kidney-shaped pond in my backyard that's around 50 - 70 gallons and has had a variety of wild minnows in it this year; at my last count in October the total was 18. They were all caught locally in streams in my area and are mostly Creek Chub but I believe there are also a few Dace and Shiners in there as well. Those 18 minnows have been in there since this summer and I haven't had any deaths all fall or winter until now. In the last week and a half or so 5 of the fish have died due to reasons unknown. It's been a single fish each time, just floating dead at the surface with no visible markings or anything else out of the ordinary (other than it being dead). Here's a recent photo of the pond so you can get an idea of the environment they're living in:

I live in Maine so obviously it's gotten a lot colder here lately and is below freezing most of the time but there's a de-icer in there that keeps the water from freezing over and these are local fish that survive the winters here in the wild just fine so I don't think the cold is what's killing them, plus I've had the same kind of fish live through the winter in there before without issue. Nothing else has changed in the past week and a half as far as I can tell. They typically hang out near the bottom of the pond during the day and then when I go out at night they'll all be close to the top just below the surface in a group, remaining fairly still. I'm not sure what that indicates, if anything, but they've been doing that for a while as well so it's probably unrelated to the deaths
Two other things I should mention: the other day I saw one of the fish sort of half floating/half weakly swimming around on its side against the side of the pond and it had blotchy patches around its head and tail that were lighter than the rest of its body. I fished it out with a net and put it in a clear container inside my house to examine it by which time it had perked up and was swimming around like normal. You couldn't really see the patches while looking at it from the side, only from above while looking down at the top of its body. I put it back in the pond and haven't seen any similar activity since then. The other thing is that one of the times when I found a dead fish in there was after I had been away for a few days and when I found it, the fish was missing its head. I'm not sure what to make of that other than maybe the other fish eating it...as you can see from the picture, there's a mesh screen fence around the perimeter of the pond that keeps predators out and I don't know what predator would eat just the head and leave the rest of the fish in there anyway.

If I can think of any other details I'll add those as well, but that's all the info I can think of at the moment. So far I haven't been able to come up with any reasons why this is happening, unless maybe the de-icer is burning or injuring them somehow, but it says it's safe for fish ponds on the box. So what is the problem likely to be here and what can I do about it?
Thanks in advance for any help!


Aug 29, 2007
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Fallbrook, Ca USA
Minnows seems to be a generic term. If they are not carp, they may be mosquito fish, who are live bearers. Their lifespan is 2-3 years.
Nov 11, 2010
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Chicago Area
Hardiness Zone
United States
That seems like a lot of fish for such that size pond. In the wild there is a lot more water per fish. They may be oxygen deprived. That would explain there sudden deaths. You can try pumping some air in the pond with an airstone or taking some fish out.

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