goldfish in small outdoor pond

Question
I live in central SC.  REcently purchased a vinyl pond(75 gal.) & filter outfit which is supposed to be efficient for up to 1000 gal.  We filled it(city water) and let it sit for several weeks when I purchased 5 goldfish(1-3 in.).  They did well for 2 wks, then overnight all 5 were dead.  We took a sample of the water tothe pet store where they said it was fine.  We have since taken to testing the water.  It is difficult to keep the ph at 6.5.  Is this fatal if it fluctuates between 5.5 - 7.5? How does one get rid of the greenish water(pond is 18") will the greenish water kill the 5 new little goldfish, which I purchased from a dealer with an outside pond.  The are not Koi, but comet variety, which I was told is what should be outside.  How does one lower the ammonia in water?  Hope you can help.

Answer
I'm sorry your fish died.  pH fluctuations can be easily fatal.  From 5.5 to 7.5 is a huge daily swing if that's what you're getting.  Algae can cause pH swings.  Add some baking soda to help stablilize the pH and raise it a little.  For 75 gallons, try adding a few tablespoons at first.  Check the pH and hardness and alkalinity if you can the next day and add a little more if needed.  Then, once you get to pH 7-7.5, don't add more except with water changes (if rain pushes water out, that counts).  As for dealing with algae, see my page on that at http://www.fishpondinfo.com/algae2.htm
Goldfish including comets and koi can all live in ponds.  A 75 gallon is really too small for koi though.  As for ammonia information, see my water chemistry page at http://www.fishpondinfo.com/chem.htm
You need to provide good biological filtration to help bring down the ammonia but my site lists some quick fixes for emergencies as well.  Good luck!