sick (and dying) goldfish

Question
Thanks, Tom, for all your effort concerning my problem.  As I've recently changed the water so frequently, I've found an area along the rim of the pond that is low so when the water is high, the ground gets wet.  It would work the other way also: when the pond is a bit low any garden water would run in.  Since my fish die only in the fall, it doesn't make sense, but it is something I will take care of.  
For now, I will think about emptying the pond and refilling it before the weather gets any cooler.  Do you have any thoughts about completely refilling the pond in the fall?
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Followup To

Question -
Thanks you, Tom, for answering quesions and helping like this.  I will leave in an hour to go out of town for 24 hours.  Will take my laptop, but don't know if I will be able to get access.  
No more fish have died.  I'm doing frequent partial water changes and the remaining ones seem to "perk up" for a while after a water change.  I've worried that the frequent changes will be too much for them in a weaken state, but it seems to be the only thing that is helping right now.  
I will try to make a list of plants while I'm away but will wait until my return, 9/25, afternoon, to check and make sure I've remembered everything.

Thanks, again.
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I forgot to say earlier, the fish show no physical symptoms.  No visible parasites, no red gills, no spots.  Nothing that I can see.
I live in northeastern Florida, the Jacksonville area.
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This is the 3rd year in a row that my goldfish have gotten sick and died early in the fall.  My 5 goldfish (5-7") are just lying on the bottom of the pond most of the time. One died about a week ago.  (I immediately ordered a pond test kit.)  A few days ago they were sometimes under the falls (as the dead one had been) and just occasionally flashing. After the one died, I added pond salt, 4.5 lbs for a 300 gal. pond, added gradually over 3 days.  Two days later when there was no change, I added 1/3 cup Jungle Pond Parasite Guard.  No change.  Today the test kit arrived, everything tests out pretty well.  The salt is high, about .26%.  The pH was a little high, about 7.5, but the booklet says that is within acceptable range.  Nitrites and Ammonia tested 0.  
The trees overhead are not deciduous so they lose leaves throughout the year, not in the fall.  There is a cedar; would it cause problems?
What should I do?  What might be wrong?

Answer -
Hi Jackie,
Thanx for your question.  In which county and state do you live?  Or, are you in Canada, Australia, New Zealand, South Africa or some other country?

thx,
Tom

Answer -
Hi Jackie,

thanx for your question.  Please try to name every plant that is growing in your yard.  Additionally, do any of your neighbors have anthing unusual growing in their yards?  Are there animals that visit your pond?  If your pond testing kit is showing you are pretty much normal then the answer is that there are either plant materials or animal problems.  I await your response.
Tom

Answer -
Jackie,
One other thing, is it possible that something is running into the pond when it rains?
Tom

Answer
Hi Jackie,
Thank you for your continued support of AllExperts.  If you have discovered a runoff, I would try to control that first before completely emptying the pond.  My reasoning tells me that if some toxin is being fed into the pond, the logical thing to do is to stop the toxin from entering the pond.  Channel water off somewhere else so it doesn't get into the pond.  Something to think about.  Tell me you city and county and I'll give you some contact information for your local horticultural agent.  The information S/he has is free of charge.  I hope this helps.
Tom