dying goldfish

Question
QUESTION: We moved to our home in a rural area of Vancouver Island six years ago and bought 40 goldfish to live in our big pond.  The pond is about 15'x 20', and 1'-3' deep in the middle depending on the time of year and how full it is.  The day after we put in the fish, only ten remained (we assume from either the blue heron or garter snakes).  After that first attack, though, the remaining ten thrived, and even started producing offspring.  We had easily over a hundred of varying sizes and colour!  We stopped feeding them fish food, as the water was incredibly murky, and they continued to thrive, until this past year.  We have never had a filter or aerator, but have kept the water fresh in the summer time by running the hose daily, and the fish love to play in the fresh running water!  We have had our share of run-ins with the Dirty Blue Heron and the Stinking Kingfisher, but I am thinking that our main problem now is Bullfrogs!  We had one big frog two years ago, and now there seems to be at least 20!  Since the first frog showed up we had a 'floater' every couple months, now I'm scooping out 2-3 every day!  The dead goldfish have mostly had no signs of being sick or injured.  I've managed to catch most of the frogs and freeze them (haha!), but I'm just wondering what else to do.  I get endless hours of enjoyment from the fish, just want to know if I can save them.

Ceilidh

ANSWER: Check the ammonia level as well as the pH.  Frogs urinate in the water, raising the ammonia and lowering the pH.

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

QUESTION: arrrgh!  Ammonia and pH levels are fine, but I don't know if I corrected them before testing by adding so much fresh water to the pond!  I have also managed to remove most of the frogs (also several days before testing).  So now I must wait and see...I also threw in some parasite ridding tablets, don't know if that will work, as the fish look healthy right up until they die...they get pretty slow, then they kind of tip and lay on the bottom...
Ceilidh

Answer
It seems like everything is in order.  It is possible that there are pH swings.  Eliminate this as an option by testing once in the mid-morning and once in the evening to compare for differences.  A slight pH swing every day could possibly cause these symptoms.  Is the fresh water from the municipality or a well?  If it is from a the city/county, you need to dechlorinate.  Check back in a week or two to let me know of any fluctuations or progress.