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Koi Breathing Issues


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Hi everyone, I'm having issues with my koi. We had 5 koi in a 125, 6ft long tank till we could build our pond, which has been running consistently for over 2 years now. So my issue is I did a water change lastnight and after that they were doing great and were pretty active as they usually are after a water change. About an hour after that they started to act lethargic and breathing heavy, more just hanging around than actually swimming at this point. So I started to dig in a little to see if I could spot an issue I might have missed or accidentally caused during my maintenance.

Temperature was the same as before I removed water

Nitrates were 20-40

Nitrites were zero

GH reading between 75 and 150, that's the two colors it's between on my test strop.

Chlorine read 0

KH reads between 40 and 80

pH reads about 7.2

Ammonia reads at .5

I typically do a rather large (75ish %) water change on them roughly every week or 2 depending on water quality and clarity. I did use Tetra brand aquasafe dechlorinator which what I've always used. 

Now this morning when I woke up my smallest koi was floating dead at the surface and another smaller koi was laying upside down on the bottom clearing having major difficulty breathing. I got him out and into my 20 gallon quarantine tank with aquarium salt currently at 1tbsp per gallon to start. My remaining 3 biggest koi ranging between 15in to 12 inch are swimming around fine but still breathing noticeably faster. I have tested my water yet again and my results were as they were lastnight when I initially tested. 

Everywhere I read online states that oxygen levels are probably my problem, but my issue is ever since my koi showed much of any growth I have always made it a priority to have extra oxygen in the tank. I used to run 3 sponge filters on the tank, I did remove one yesterday when I did the water change, but I also have a large airstone disk that has been running for easily over a month before this water change. I also have 2 of the big tetra whisper double HOBs running on this tank and two smaller ones that I've ran.

I'm just confused as to what else could be wrong or what else I should do. I've considered doing yet another water change but I worry that wont help or could cause more issues. So I'm just putting this out as feelers to see if anyone has any advice or experience that I'm lacking. 


Sorry this was definitely long winded but thanks in advance to anyone who stops to read and/or provide insight.



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I'm going to say that the ammonia is definitely an issue. .50 is too high. They may be getting to the point where they are too much bioload for that tank to handle. I'd up my water change schedule until you move them to your pond. 

I'd make sure to do another large water change, check all the filters to make SURE they are running properly, add more air even -if I have something on hand. Make sure I'm dosing the water treatment correctly. I'd probably add salt to help gill function. I suspect ammonia poisoning. 

Edited by xXInkedPhoenixX
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Sounds like a combo of low oxygen and ammonia. A lot of fish can survive very low oxygen as long as there's no other stressors. The ammonia might be that stressor. Plus, ammonia itself makes it harder for fish to breath. Koi consume a lot of oxygen, at a certain point no amount of aeration can keep up if they don't have a big enough tank.

I'd do everything @xXInkedPhoenixXrecommended + reduce feed, lower the temp if possible, maybe reduce lighting so they're less active.

I used to have the same amount of Koi in an outdoor 300 gal. An animal ripped out the air stone tubes one day and the fish had the same symptoms you're describing with the smallest dieing first.

If you look at their gills and they have a blue-ish tint, the gill filaments are sticking together, or there's bleeding around the eyes or on the gill cover, those are other signs of low oxygen. A big discolored spot on their side or their mouth stuck open after death are other signs.

If their gill tissue, the base of the pectoral fins, or a spot on the top of their head is bleeding or if they kind of spaz out every once and a while, then you know it's chronic ammonia exposure.

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