Well, how about letting us know what the pH read before and after the acid was added. You probably didn't need to do anything. How did you measure the pH?

Pretty sure the number one rule is test your water before you do anything otherwise how will you know which path you are on. Please get your water tested. It's very important to know that number. I use API freshwater master test kit but you can buy just the individual test kits at some of the big box pet stores or garden center if they support ponds.

I am using test strips. I have never been able to get a reading below 8.4 ph. The alkalinity is always at least 240. I have tested the water before adding vinegar and after. These numbers do not change. The photo below was taken right after the pond was built.

There is nothing wrong with your pH. Goldfish and Koi and many fish don't mind high pH. Your plants are also OK with the pH. Have you fertilized your plants? I use Jobe's tomato spikes. 1 or 2 per container every month or so. The high pH stabilizes the pH. Read about using sodium bicarbonate to buffer your pond water from pH swings.

I just got an API pond master test kit plus the tests for GH and KH. The KH is 19 the GH is over 26 and I tested it twice and the wide range pH is at 9. This is a 6,000 gallon pond filled with well water. I also bought BOTANICARE pH down but have not figured out how much to use yet. Thoughts?

Same thoughts as before; don't worry about it. A pH of 9 is OK and explained by high concentration of slightly basic salts which indicate that you wouldn't be able to change the pH much anyway. See if you can return the pH down thing. AND, from your picture, you have a 600, not 6000 gal pond.

Agreed, don't worry about it. I add sodium bicarbonate to bring the pH up to over 8 and to buffer the water from sudden pH swings due to heavy rainfall which could kill the fish. I suspect to get your pH lower you would have to dump a really high amount of the pH down into your pond and it won't help for long and it will make your pond susceptible to pH swings. Your pH is fine. Leave it alone.

Consider adding a biofilter to your pond. It will help with the plants and improve the water quality for the fish.

Still worried about this. I added some gold fish last summer and in 24 hours they had disappeared. I assume they died and sank to the bottom. Water lettuce has died along with most of the plants. Even some reeds I transplanted from a local lake are brown in the tips. Any none of this is cause for concern?

My water from the tap is 8 or higher. What is the ph of your well water? My aquarium fish are fine with my water so I can't imagine that you have issues for fish or plants at 9. Could the small fish be hiding in the rocks?

I never have much luck with floating plants, I blame lack of nutrients. Could it be the lower light levels of approaching winter causing the plants to start going dormant? I don't know if San Diego gets cool enough weather to have them go cold dormant. How did your plants do in the summer and early fall?

I don't think you had dead fish. Dead fish float!

I am more concerned about the plants now that I am reminded that you are near San Diego. Even at this time of year I think water cress would grow. Go to a fancy grocery store and get a bunch from the produce section and stick some directly into crevices in your stream. It wants flowing water. You could also go to a pond store and see if they have Sweet Flag (acorus) and get a pot of it. You can break that pot clump up into several pieces. Put a couple pieces back into some soil like stuff covered with kitty litter or gravel and set those pots in some pond water and cram a couple sections without dirt in stream crevices like the water cress. If they die, I think I would be worried, but of course your pond looks beautiful anyway. Most people around here are worried about grungy algae.

If they grow well, the root masses will be wonderful filters. In fact acorus could be a problem to keep an eye on. It can be hard to remove after a couple years. Water cress is easy to remove. You can eat it too. It may get too straggly looking 'though perhaps not in your zone.I am suggesting it mainly for test purposes since it should love your stream.

If you don't get a couple gold fish after some plants get established, then you will have to get some fertilizer. I don't see why you couldn't just throw some right in the water. With fish, sometimes people are worried about too much fertilizer although they are still often cramming stuff into their water lily pots. It is a balance thing. I have so many plants in my stream that I should probably be fertilizing my lilies like crazy because my stream root masses are probably taking everything away from the lilies that didn't flower much this year.