When you say you had your water tested, what did you test for?

Ammonia, nitrates, ph

What sort of different or strange behaviour?

Laying on the bottom of the pond on their side...randomly jumpingl and sucking air at the top of the pond.

Laying on their side is a sign of extreme stress. Gasping at the surface is a sign of low oxygen.

Do you have a lot of decomposing organic matter in the bottom?

Do you have aeration?

950 gallons is barely big enough for one full sized koi. How big are your fish? How much filtration do you have?

I am very sorry to hear about your koi fish. I, too, have experienced similar deaths of my goldfish over the past weeks, and I attribute it to a bad batch of frozen fishfood. The symptoms in my goldfish sound identical to your poor koi. I would be very interested to hear if you have recently bought frozen fishfood to feed your koi.

I elaborated on it more on this other thread by another person who has also lost fish recently. http://forums2.gardenweb.com/discussions/4702258/why-are-my-goldfish-dying?n=5

Also, was it unusually hot?

We haven't bought frozen fish food. That's terrible to hear that the food caused your fish to die. Yes, it was unusually hot, and I didn't know to add water on those days. My husband cares for the pond, but he has been traveling A LOT lately, so I do take some blame for the fiasco. But, I feel like there had to be something more than the heat. Thx to everyone for the feedback. My husband will be draining it, cleaning it, and we will start over...

Cold water holds more oxygen than warm water. Warm temperatures also cause bacteria to kick into high gear which, if they have food, uses up a lot of oxygen. Put the two together and you have the conditions for very low oxygen levels that can kill fish. Bigger fish need more oxygen, so that's why your oldest fish died first.

For a pond of that size, I recommend going for goldfish instead of koi. There are plenty of really awesome goldfish out there that look like koi but instead of 20-30" long they get 6-10" long. http://www.samurai-goldfish.net

Just ignore the whole, "poor man's koi," bs.

What sort of filtration does the pond have? This could be an opportune time to make upgrades.

Can you send a picture of your pond? When it is hot, I add extra aeration, so they get more oxygen.

This all makes a lot of sense now...thank you...we did buy an aeration rock...strange though, we've had very hot weather here before...however my husband wasn't traveling so maybe he does just automatically add water. Interesting, we have had 12 fish in there for the past 15 years.we've never been told that it was too crowded. Nor, has it ever been an issue.

I m very sorry you lost your fish - I know they are beloved pets - but pls, hear me out here - I too had to learn the hard way that not everything the pond salesman told me was true... it was truly heartbreaking...

The salesman wants to sell products, and while some are better than others at providing factual information and good advice.... it still is essential to do our own research... the info is all readily available these days via some pretty good koi forums (not the one on GW!) and koi-specific websites.

Whats typically recommended is 1000 gallons per fish for the first koi, and a minimum of 500 gals for each of the others - 1000 per fish is optimum really - and they need at least 3 or 4 feet of depth for diving and good health... after all, these are fish that reach lengths of 2-3 feet - its a total myth that fish stay small to suit the size of the container. (Poor health may stunt their growth to some extent however.)

The whole "inches of fish" thing is absurd too. A fish 20 inches long is not twice as big as a 10 inch fish - its more like three or 4 times the size, being wider, heavier, eating and contributing much more waste to the system. Its not surprising that things were fine for a long time but then eventually weren't.

One factor contributing to oxygen depletion that no one has mentioned is excessive organic waste decomposing - yes the warm temps would have exacerbated the situation, but the root cause (given your crowded situation) is likely to be the water conditions. This could explain why hot weather before didnt have the same impact it did now.

I guess I would reiterate Nils question re: what your bio filter is, and also what were the results of the tests. Ammonia and nitrite should be ZERO, nitrate should be very low if any.

Finally it should be noted that crowded conditions or poor water quality can also make fish more susceptible to disease.

I also second the suggestion to get goldfish - you could have 9-12 happy fishies in that size pond. I have 9 in 600 gallons and I assure you they are every bit as loveable and fun to watch as a koi. There are some with long flowing fins that look just like koi.

Again I am sorry for you loss - pls understand my intention is not to make you feel bad, just comes from concern about peoples fishies.

I have a 4000 gallon pond and 8 koi. I don't exceed that number.

go out to the pond in the very early morning and see if the fish are gasping at the surface. That is a classic sign of low oxygen in the water. Unless they are too stressed to even get up there....You may have been fine with so many fish in your pond for years until you aren't. Basically, you got away with it. Usually the biggest are the first to go.