Small koi dying, big ones ok. Odd?

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Hi everyone,
First time on this site, looks like a lot of great info and resources available.
Anyway I'm at a loss, I have a large pond probably 6000 gallons with all koi from 1 inch fry to 12inch. Qty of fish is about 14 fish not including fry from 1-3 inches, they hide well.
Pond has a sand filter, waterfall, skimmer and large bubbler that all are still running as of 1/12/17.
Big koi are fine but smaller fish are dying off, I'm in NJ so our winter has not been bad and the pond hasn't frozen over.
Any ideas would be greatly appreciated.
 
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Welcome Nick40, I'm sorry the loss of your small koi brings you here. Do you test your water, for ammonia, nitrite and nitrate? I'd probably start there, I'd also be checking the water temperature...not air temps. Have you had your pond while, in other words, it's established?
 

sissy

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Welcome and I came here from NJ .Have you checked your sand filter as they can get toxic in the winter when there is no bacteria in them .I saw a video on you tube where a guy talks about his sand filter and why it was no good come winter .He built his own filter after loosing most of his fish .I remember the video but can't remember where to find it again .
 
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Small Fish dying but Big Fish surviving is usually a toxin. Did anything get sprayed around the pond or in the area brought in by rain?
 
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Welcome to the group, so sorry for what's happening to you:(
 
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Wow, I appreciate the replies.
Pond is new since April, I built it myself the first time but hired someone this past April to build a much bigger pond. Temp is not something I've thought about and the pond is 4ft plus deep. The pond in my opinion is well established since the bigger koi spawned successfuly.
The sand filter is new to me so it becoming toxic in winter is something to think about. I usually shut my filter down in winter and just run a bubbler and heater to keep a whole in ice of it forms with no problems.
Toxin is always possible but unlikely, I don't spray or fertilize near the pond ever.
Water quality I will check when I get home this evening but typically I don't have an ammonia or nitrate problem.
Stand by, I'll post again this evening.
Thanks
 
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Nitrite 0ppm mg/l
Ammonia 0ppm mg/l
Phosphate 2.0 ppm mg/l
PH 7-7.5
Water Temp 45F
Since nothing stands out to me as real bad I'm gonna guess toxin. How would I remedy that besides draining pond, activated carbon?
 

sissy

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Could use quilt batting with a hose from your pump running into it > set mine up that way again today because after snow melt the water got cloudy
 

Ruben Miranda

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Hello
If your filter or sand filter went toxin ok would think you would have ammonia level and nitrite go up (hopefully someone would verify this)

If you can (i know it is winter)
I would do a water change with decor and run some activated carbon.
Depending how much carbon you use I would run it for a week and then replace it for fresh.

Ruben
 
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Definitely add activated carbon after a water change to ensure the toxin removal.
 

Meyer Jordan

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Go with the Activated Charcoal (at least 8 ounces). I would belay any water changes. If any toxins are indeed in the water column they need to be removed not reduced as with a water change.
I really suspect that there are no toxins in play here. If the fry are indeed fry and not fingerlings, i truly suspect that temperature has played a large part in your problem since the ideal temperature range for fry is 23C/73F - 30C/86F.
 
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im wondering if toxins , all the fish will show abnormal behavior , either there gills or breathing will show some signs or swimming behaviour , u need to observe fishes for any disease too which effects smaller fish 1st usually all smaller fishes die in some diseases

Algae bloom , diseases . temperature and water quality they effect smaller fish 1st .
 
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Fish Kills Chart
 

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