Yikes! Sorry you're having such a rough go of it. I have the same size pond as you, and sorry to say, but I haven't had any major problems with it.
What kinds of pumps are you running on it? What are the dimensions? Please don't take this the wrong way, but could you maybe making this a bit more complicated than it needs to be?
Any expert and people you hire is going to get you to pay through the nose at any oppourtunity. Buy the biggest pumps, the "superior" set up. I'm running a 40$ pump out of a filter I made using an old waterjug (like the kind on watercoolers. Pop in some holes, drop in some stones and quilt batting, works like a charm. Toss in a water lily and clean the whole thing out once a year.
Perhaps try and simplify things for future.
As for your fish showing distress right now, it could be a fungus or disease, it could be ph level, it could be ammonia level, or possibly something leaching in and contaminating it. In other words, without seeing the fish or knowing the symptoms, I wouldn't even know where to begin. Sorry!

Dave, are there any water garden clubs in your area? If there is, you may inquire if they will clean it for you.

Happy ponding,
Scott

Dave if you have a digital camera why don't you post some photos so we can see what you have there. Maybe with them one of us will recognize a problem and be able to suggest a solution. It is worth a try.

Here is a link that might be useful: My pond

Thanks everyone. I'll try to post pics, and supply more info, but right now, I am so slammed with other priorities (I realize I may be burned at the stake for not making my pond the #1 priority in my life), that my only hope is to find someone who cares for ponds professionally, to address the issues, and tide me over until I can take over and get back into the flow of things. I'm really hoping someone can recommend someone in my area to do this work.

Thanks to all who are offering their sympathies and help.

I've done a 50% water change, and the fish seem very much better, but this is not a solution in the long term. Or is it? 50% water changes ever so many days?

Dave

The water change may have done it unless you have mitigating factors that caused the PH to crash in the first place, in which case, they should be addressed. You should also consider a buffering agent to help stabalize the water. That's the most important thing is keeping the water stable.

Personally, I try to stay away from chemical alterations if possible, particularly PH up or PH down. You don't want to create a system that relies on the chemical to keep the water even and could crash on you without notice.

I hope the water change did it, you didn't say what the circumstances were that caused it to crash in the first place.

A lot depends on the kind of fish you have, how many, how big, etc. If you have a lot of koi in that size pond, you're asking for trouble (politely). Same goes for goldfish, when it comes to fish load.

Also, if you can't see your work load lightening up in the near future, I don't know that I disagree with your wanting to let this particular hobby go. I don't have koi (only mosquito fish) for one main reason - they need the utmost care of their environment to not only thrive but to survive. And as with any pet, they need attention when they become ill.

I'm not judging you, just so you know. But I think you realize, especially with your current schedule, having a pond with fish isn't the best idea right now. :)

Brenda

Destany: My Pond is buffered, so it is a mystery to me why the symptoms match the pH crash theory.

Brenda, Thanks, I am not feeling judged. My problem is not that I never have the time. 99% of the year the koi are happy and healthy, and my time is much more open. But during the first warm days of spring, things always seem to go suddenly haywire, and I my time tends to be swamped that time of year, and the emergency always seems to happen suddenly and unexpectedly (as emergencies do). Does that mean, that, because of a 1% time-frame crunch with problems, I should give up on koi the remaining 99% of the time? Why is it so hard to find someone who does this professionally to tide me over the tough times?

First of all you are asking people hundreds and thousands of miles away from you who you can get to help you in your area. Everyone wants to help, If I could I would drop everything now and come to your rescue. I had the same problem with my daughters pond around Christmas. We took all the fish and frogs out of the pond, put them into a clean large trash can, completely emptied her pond washed it out, refilled it, cleaned the filters, put the fish back in and added some salt and her fish recovered. She only lost a few. Of course they aren't Koi. It really didn't take that long to fix it. Maybe you could ask around any place locally that has fish, maybe put some messages on bulliten boards in Petsmart, Wal Mart "Oops, forgot you don't have many Wal Marts there." Maybe Petco or Lawn and Garden Stores.

davevallejo,

A lot of koi clubs provide assistance in these situations, so you should contact a member from the link below.

LP

Here is a link that might be useful: San Francisco Bay Area Koi Club

pikecoe: Thanks for the ideas. I do realize that this is a global forum, but my (misplaced?) hope was that there'd be enough people from my general vicinity that someone would say, "Yeah, try so and so."

It looks like I may have found a good guy (an aquarium expert) who can come next week, so hopefully I can take care of the immediate issues. Ultimately, however, Brenda will likely be right: I will have to depopulate my pond, or outright turn it into a mosquito-fish-only pond.

This is obviously a hobby for people who never have busy schedules, something I used to think was a rarity in this day and age of multi-tasking overload.

Dave, I'm really glad to hear that someone is coming over to help you.

I think that if your pond were larger (go for it! LOL), and your filtration and aeration were adequate for pond + koi load, you would find healthier fish and a lot less maintenance involved. The smaller the pond, the greater the limitations.

Let us know the verdict and how things turn out!

Brenda

I have a final diagnosis which I think is accurate: Hydrogen sulphide poisoning.

The aquarium guy is telling me I have to give up all but a couple of goldfish (goodbye koi), because it is impossible to have koi in a 500 gallon pond.

I think at this point, I'm going to turn the pond into a rock garden, and give up on fish.

Dave, I'm sorry to hear that. It's good you have an answer to your questions, I bet that's a huge relief. Sorry you have to give up ponding, but it might just make your life easier. I have 2 feeder goldfish in my pond - now 7 inches long and three years old, lol. Just those 2 little guys are quite enough for my enjoyment and won't overload the pond. Good luck with your rock garden!

Well, at least the aquarium guy knew what he was talking about, Dave. I was afraid you'd get one of those folks who says you can have as many fish as you can cram into your pond. *shudder*

As mentioned above, you can still have beautiful goldfish in your pond, and save the koi dream for your next gigantopond! :)

Brenda

Dave:

I have had aquariums for about 40 years and got into ponding 3 years ago. There are similarities and there are differences. My pond is 1100 gallons with 9 large goldfish and a population of about 20 rosy reds. My first year went well until I found dying fish. It turns out that, even though I was cleaning the filter material in my prefilter, there was a big glob of muck at the bottom of it that was emitting hydrogen sulfide that caused the problems. When I started cleaning this out, I stopped having any problems (except for the Heron but that is a different story).

With a 500 gallon pond, you can probably handle 4 to 5 mature goldfish and some mosquitofish or rosy reds. The rosy reds eat bugs that land on the water and I assume mosquito larvae because I never see them.

I know a guy who has like 6-8 koi in a pond that can't be over 250 gallons [one of thoes hard plastic liners]. He's had them for years [one is well over 2' long]. He uses a deicer in the winter as the pond isen't very deep. It's seems like you should be able to keep a few koi in a 500gal pond. Hydrogen sulphide gas usually emits a foul odor [rotten egg smell]. I assume he checked for ammo, ph and nitrates. Perhaps your filter is not working properly and the water is not getting enough oxygen. Overfeeding would make it worse.