Pond fish keep dying

Question

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Followup To

Question -
I have a 200 gallon pond in my backyard with black plastic liner, river rocks on the bottom, 2 lilly pad and 1 iris plant, a filter with a water pump feature that acts as a waterfall and it has plenty of shade. I started off with 2 Koi, 2 shubunkin goldfish, 1 butterfly plecostomous and 1 regular plecostomous, they were doing great until my sister introduced a red ear slider turtle into the pond, he ate all of the fish with the exception of the 2 plecostomous. Since then I haven't been able to keep the fish alive in the pond for more than a few hours. I bought new fish the day the turtle ate them and introduced them into the pond. They all died including the butterfly plecostomous. I have tried twice since then to keep other fish alive, i even tried little feeder fish, and none of them seem to live for more then a few hours. I still have the regular plecostomous and he is still in the pond and doing great. I have checked the water levels several times and have taken water to have it tested and everything has come back fine. I have talked to a few people and no one can figure this out I am kind of at a loss as to what to do next.
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The koi that I started out with were about 6-7 inches in length and the shubunkin goldfish were about 3 inches and turtle ate them, i know this because I walked out and saw him in the middle of eating the koi.I removed the turtle immediately, he has not been in the pond since that incident, we keep him in a different enclosure. My problem was that since then, every time i add fish they dont live more than a few hours, they start swimming in spirals after a few hours and slowly sink to the bottom. When i tried adding the little feeder fish after i took the turtle out to see how long they would survive, the larger ones acctually lived over night and halfway into the next day, which was an improvement from 5 hours. the plecostomous is still doing great and all of the plants are doing fine

Answer
Oh, that's strange.  It sounds like something is wrong with the water unless you've gotten bad batches of fish each time.  As I mentioned before, I suggest water changes, adding carbon to the filter, testing the pH, ammonia, nitrite, and oxygen levels, and perhaps cleaning out the entire pond.  There may be something in the water or pond that is making it toxic.  It can be hard to figure out what that is.  I wish I could help more but I'm here, and you're there!