Sorry to hear about your fishie loss. It is so hard to lose them and especially when you don't know why.

Anne

I assume you checked the fish for any signs of disease or wounds after they died? I would suspect parasites at play but your Koi seem to be unaffected. Normally Koi are less hardy than goldfish which makes this problem rather puzzling. Did you not expand your pond this year? Could some parasite possibly have been introduced with new plants or fish? Hope you solve this, losing fish can be fustrating.

What about water temperatures?

Fish look fine when pulled from the water, other than the fact that they are dead. Eyes are bright, coloration is normal.

I had about a month ago a bad case of fin rot on my largest Koi. I treated the pond, and fed them an antibiotic food for 14 days per direction. I did not use a fluke treatment, as it specifically said not to use if you have invertebrates (I have crayfish).

The Koi, and the other fish that had fin rot have rebounded nicely and the fins are filling in.

I'm not sure if the food caused my bacteria in my biofilter to die off, explaining the ammonia/nitrate spike.

But the fact that the Koi are doing well, and the goldfish are dying is puzzling.

And to the person that asked if I expanded my pond, I did. I haven't introduced any new plants/fish since then. So as far as I can tell, thats not a big issue except for stress on the fish. But that's been 8 weeks ago, so I'm not sure if it would still be causing trouble.

Forgot to mention that watertemps are normal. I'd say they are in the mid 80's. I don't have a thermometer in the water though, but it's not really warm/cold when I work in it.

At 1800 gallons I don't have a big fluctuation during the day, and the pond itself is shaded about 50% during the day with a large tree.

It seems that my PH is crashing. I broke down and visited a pond shop here in town for some expert advice.

After leaving with some answers, I did a bit more searching. I think I'm going to begin adding baking soda to the water to buffer it from further crashes.

I seem to be currently trapped in a viscious cycle of ph crashes killing the bacteria in the filter, then spiking levels of ammmonia and nitrites.

Lost 2 more large goldfish, bringing the total dead of 7 large goldfish, and numerous minnows.

I'll update as the situation progresses.

Sorry you lost more fish. Baking soda will work for a quick fix, but crushed oyster shell will be better in the long run. The buffering will last longer by dissolving slowly as the water flows over it. Surprised you never had this problem in the past.

Where does one get crushed oyster shell? I haven't seen them in bags at Sams!

Seriously...where would I go for it?

I get oyster shell for my chickens at Tractor Supply. You could probably also find it at other farm/feed stores.

I got a 4lb box of arm-n-hammer for $2.69. Pond has stabilized. I put in 1 lb on Monday, and 1 lb on Tuesday. PH is holding at 7.5. Fish are responding very well, swimming around much more now.

I also 'reseeded' my bio filter with some bacteria, and added some slime coat to the water.

I didn't realize at the time, but my alge was all dying off as well. It's coming back quite nicely (on the walls) and is food for the small fish.

This weekend I'm going to be getting a bag of crushed coral/oyster shells and put in the water to help buffer it.

I don't want to jinx myself and say I've fixed everything, but the pond seems to be on the mend.

Great news, did you happen to test the PH and KH of your source water?

PH is actually fairly high (around 8). But the KH is appearantly low, as it kept crashing.

What is the source of your water?
Mike

There is a good possibility that water conditions became poor maybe due to a filter not large enough for the size of the pond or the fish in it. If you don't make monthly water changes from the bottom of the pond where toxins congregate, this can also cause problems.
I've got a great diagram of a bio-filter which is easily made and maintained and you never have to clean the bio-media and our pond water is now always nice and clear and we don't get hair algae any more and our fish are always healthy. It has been suggested by the Japanese that if a pond is overstocked, you should use a 45 gallon drum for filtration for every 1,000 gallons of pond water. I heard this many years ago and it always seemed to work well for us.

Your's Koily, Lorraine

Water source is municipal water (not well).

Filter size is adequate to 'over'. I clean it regularly (2x weekly in warm weather) and only rinse the pads with pond water. No hose water that would kill the bacteria in the media/filter.

Everything is doing well now. Fish are lively and eating very well now. I added some crushed coral to both my waterfall area and the filter. PH is holding now at 7.5 and doing fine.

Pond is returning to normal. Thanks for everyone's help and concern.