top of page

Ammonia Spikes with UV Light after Pond Setup

Updated: Jul 8, 2021

Ammonia spikes are dangerous and a terrible event that can lead to the death of koi.  

I know from personal experience that, when you're putting a new pond together, everything seems to be right on for a couple weeks. Then, when you add your fish and begin to feed them regularly, you may not have much of the ugly green water that defeats the purpose of buying beautiful koi.  

Many suggestions are out there for combating view-restricting algae when it appears. The most popular that I know of are UV clarifiers and sterilizer, and chemicals.  I happen to have two very high-strength UV lights.  When I set up the system I was not really using them because the water was so clear.  

Also, you should know that when I set up the new pond I had left them off so that the beneficial bacteria could take root.  

After about a month, I noticed the green shadow of algae taking root in my pond. At this point, I turned on the UV lights. I couldn't help but notice the spike in the day following this.

The ammonia had appeared in extremely high over night.  So my suspicion was that the lights may have affected the algae and the beneficial bacteria because when I turned them off the ammonia was still climbing. The ammonia production was running uncontrolled by good bacteria.  


It all came down to a disruption in the nitrogen cycle. Fish eat, produce toxic ammonium (NH4+), and good bacteria (nitrosomonas and nitrobacteria) eat that ammonium and produce nitrates (NO3-). Nitrates aren't toxic to the koi and can be processed by your plants or algae.

Leaving the UV light on over night had completely killed off all those nitrosomonas and nitrobacteria, so the ammonia was never converted to nitrates. All that toxic ammonia was just floating in the water, posing a huge threat to my koi.

So, now what?

After a healthy water changes, leaving the UV off, ammonia binder, and more good bacteria, I noticed things got back to normal.  So, I turned the light back on as an experiment to see if it was indeed the light causing the problem. Because there was another ammonia spike, I realized the UV light was causing the problem.  

What appears to be the cause is the death of all that algae leaving ammonia and suffocating the fish because the lack of oxygen, etc.  My next step was to completely redo every step over to get the pond back on track.  This can be a stressful event not only for your koi, but for you as well.  

It was time to phone a friend! After speaking with several people and reading on the subject I felt that I could solve the problem.  What I found was that there are several ways to solve this disruption of the nitrogen cycle.

  • One is to set your UV lights on a timer and run them for a short period so that your system is not shocked.  After all, you do not want to shock the system.  

  • You can also do water changes while you've got your UV light on a timer.

  • Finally, add a very small amount of Algaway 5.4.  And I do mean a small amount.  This will help to clear the water when you want to clear the cloud out.  Then your water will be very clear.  

Make sure that you maintain your water changes regularly and increase the light timer for your UV until you can leave it on.  This process can take a while but It is worth it.  

Enjoy your Koi and Happy Keeping!

Call us if you have any questions!

828 views2 comments


Select Koi Inc
Select Koi Inc
Dec 09, 2020

No problem! Thanks for the feedback! Hopefully it helps!


Thanks for the info! A lot of great info. Learned I was doing a big "no-no" by cleaning my filter. I have a C-2 Fluval Hang On Back Filter in a 5.5 gal Fluval Chi.

bottom of page