My big fish are dying

Question
I have had a fish pond for almost 10 years. Everything thrives in it. There are fish and frogs of all stages. The pump hasn't run for several years. I finally got the pump and umbrella-like fountain going. Even before this,my fish weren't surfacing as they always do. The neighbors had a heron swoop down and steal theirs. I assumed the same of mine. So I bought 10 new small ones. It turns out my old 8 yr old healthy fish were in there all along! And now I am losing my big fish on an average of one every other day! Until then, it has been the same 17 big guys in there looking healthy as ever for 8 yrs. Several of the new littler fish have died, too. There are lots of plants taking over the pond. It is approx. 8ft. long and 4 ft. wide with a 31 inch depth. They survive harsh winters here in Ohio. What can I do to stop this cycle?

Answer
Hello Cindie!
It is hard to say whether the new fish brought in disease or the addition of new fish overwhelmed the system and increased ammonia and nitrite to toxic levels.
The first thing I would do is test those levels. Reasonably priced kits are available readily to do this. Many pet stores that carry fish also do it for free.
If the levels are anything but zero, then the water is toxic and feeding must be stopped immediately and 1/3 water changes done daily (don't forget the de-chlorinator) until those levels are again at 0.
If, however, the levels of those two poisons are zero already then there is a good chance that one or more of the fish that were added carried in some disease.