Sudden fish die off

Question
I have had a water garden for about 15 years, and the pond in question for about 5 years.  I have tried to allow nature to balance my pond, and it has always worked, until now.  I had several hundred mosquito fish, about two dozen gold fish, innumerable toad tadpoles and three frogs co-existing in the pond for the life of this pond.  Just yesterday I noticed a floating gold fish.  When I looked more closely, I found 8.  Today I came home to find all of the rest of my gold fish dead.  I looked closely and I could only identify a handful of mosquito fish, and all of the tadpoles were gone.  It is too early for them to have matured enough to have left the water.  The frogs seem quite fine.  Since this has never happened before, I started looking for a cause.  I purchased a lily in a prepackaged container at Walmart over the weekend and placed it in the pond.  The die off started within a couple of days of that.  The only other recent change I have made is to place a barley ball in the pond.  Any idea what may have happened?  Should I drain the pond and refill it?  If so, what might happen to the toad eggs that have not matured into tadpoles?  Should I do anything before restocking? Final question, could this have been caused by me not getting my recirculating pumps going soon enough?  I have had the pump running, but only a couple of hours a day rather than the usual 10 hours.  Thanks a lot for any help you can give me, and sorry for so many questions.

Answer
Hi Bill,

Many plants are grown without having fish into consideration.
Maybe some chemicals were applied to your water lily.
When buying new plants it is always good to flush them of any possible contaminants. The best way of doing this is to keep the new plants in a separate containner and change the water every few days.
I would recomend you changing the water of the pond before any restocking.
Changing the running time of the pump will also increase the level of methane from decaying matter. A lack of oxygen will result in the die off of fish and animal life.

Good luck

Carlo