Goldfish Mysteriously Dying in Established Pond

Question
QUESTION: We have a well established pond in our backyard for approximately 3 - 4 years now.  We have many goldfish and were doing well until this summer.  In the past 2 weeks our largest 6 goldfish died.  I checked the PH and Ammonia, which turned out fine.  We have a waterfall, a small fountain and an aeration stone that I bought recently thinking it had to due with poor oxygen levels.  These fish have multiplied every year and we have many smaller ones in the pond that seem to be doing well.  Like I said before, it's the largest of the goldfish that are dying on me.  Besides bloating (bloating due to death -right?) I couldn't see anything visibly wrong with their skin (i.e. ick, fungus).  I also noticed our plants, which usually do very, very well aren't doing that great this year.  I don't know what to do at this point.  Can the pond be overcrowded?  We did a partial water change recently and have been keeping a close eye on it.  I just don't know why the larger fish are dying at this point.  Please help.
I greatly appreciate it!  


ANSWER: Hi Kathy,
Thanx for your question.  I have a few questions for you.  You indicated the dead fish were bloated.  Did you notice any bloating prior to their deaths?  I think your pond may be overcrowded.  Can you tell me how deep the pond is, what are the dimensions or better yet, do you know the capacity of the pond in gallons?  You said the plants weren't doing well.  Have you had a lot of heavy rains lately?  What kind of trees or shrubs may be dropping leaves into the pond.  Any run off coming from your lawn?  Has your lawn been treated with fertilizers and/or weed/bug killers?  How many fish have died?  Let me know so I can better answer your questions.
thx,
Tom

---------- FOLLOW-UP ----------

QUESTION: A few more fish died this weekend... two smaller fish and another large one.  No, I didn't notice any bloating prior to death.  We added a single fish 3 weeks ago, which is still healthy and swimming around.  We used a net to transfer the new fish from tank to pond.  He/she looks great.  Our pond is about 3 ft in the middle and gradually reaches about 2 to 1 ft around the sides.  I am guessing it's around 900 gallons or so.  The bottom of the pond doesn't have any rocks or pebbles - just around the sides.  Do we need to put medicine in the pond as a precautionary measure? It hasn't rained much in our area.  The Iris is doing really well, but the floating plants do not seem to thrive this year.  We have an oak tree above the pond and many others in the backyard.  Yes, leaves do fall in the pond on occasion, but we try to get them before they sink to the bottom and start to decay.  The lawn hasn't been treated.  All together 9 fish died in a 2 week period.  Stumped.
Thanks for your help.


Answer
Hi Kathy,
You described bloating and I think therein lies the problem.  If you notice any of the other fish beginning to bloat, remove them from the pond and keep them in separate containers until the bloating resolves itself or until the fish expires.  That's the best thing I can advise at this point.  You may have some overcrowding in the pond too.  The rule is generally 1 inch of fish per square foot of pond surface.  Overcrowding in the pond can lead to fish death.  You mentioned that the fish have been reporducing and if overcrowding is occurring, the larger fish will eat a lot of the babies.  At any rate, this is the best answer I can think of.  I hope this helps.
Tom