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How To Handle a Nitrite Spike
Whenever 0.25ppm of nitrite or more is detected in your pond water:
Increase aeration to maximum. For a a nitrite level of 1 ppm or higher, add suuplemental air, if possible.
Stop feeding the Koi if detected in an ESTALISHED pond, reduce amount being fed by half is starting up a new filtration system.
Discontinue use of any UV sterilizers, ozone generators and/or foam fractionators (protein skimmers).
If the nitrite levels is 4.0 or greater, it is highly suggested to transfer the fish to a "safe" holding tank.
Your #1 Line Of Defense Will Be Daily Water Change-Outs.
Make Sure To Use Dechlorinator With Each Change-Out.
Water change-out instructions:
Nitirite level is less than 1 ppm: do a 10% change-out and ADD 1-pound of non-iodized salt per 100-gallons of total pond volume.
Nitrite level between 1 and 2 ppm: do a 25% change-out and ADD 2-pounds of non-iodized salt per 100-gallons of total pond volume.
Nitrite level greater than 2 ppm: do a 50% change-out and ADD 3-pounds of non-iodized salt per 100-gallons of total pond volume.
NOTE: Very affordable non-iodized salt can be purchased at big-box DIY or hardware stores. It is sold for use in water softeners. Make sure you do not buy the salt that contains "red out" or has other chemical additives.
The addition of the salt helps reduce the toxic effects on the Koi, but you should only look at it's use as an interim measure. Address the problem causing the nitirite spike and correct it!
If this spike is occurring in an established filter ... you are doing something that is negatively impacting it. Have you rinsed out the filter media recently? It might be so full of waste and goop that the pond water isn't able to flow through it any longer. Though there are pros and cons against using the pressure from a garden hose to clean off media, I would go ahead and hose it off thoroughly to dislodge all the gunk and reinstall it into your filter.
Make it a point to inspect your filter media at the very least on a weekly basis until you get accustomed to how dirty it gets in what amount of time. Yes, you do have to keep the filter media clean ... but not overly "clean". Remember that you do not want to kill off all the friendly bacteria that reside there, the ones that process the ammonia and nitirites.
If your filter media is getting clogged with waste and such in a few days, I will make an educated guess that you need more filtration and/or better in-pond maintenance. And I will make a wild guess that you might be housing too many fish in too little of gallonage; and you might also have plants in your pond that are contributing dirt that is helping clog your filter media.
I cannot address in this article all the variables I have seen or heard about when it comes to pond and filter problems. I would be able to write 100 pages and still not touch on all the possibilities. But I have tried to give you what "normally" causes a nitrite spike and how to treat it.
Test Your Water Daily Until Readings Return to Zero
Once you get your pond back to acceptable readings, continue to monitor your pond water. Don't take things for granted ... check the pH, ammonia and nitrite levels once a week after you get your pond back on track. Continue to make sure your filter media isn't clogged with gunk ... keep it fairly clean also.
The few minutes it takes to do the tests and check up on your filtration equipment is nothing compared to what you may end up going through by becoming lackadaisical. Besides the hassle of dealing with pond problems, you could end up killing all of your Koi.
Don't Be A "Fish Fixer" ...
Please Become A Dedicated "Water Keeper" | 
