Massive koi death in pond, without an apparent reason -- need help!

Question

I am still experiencing unresolved problems.

Shortly after 5/1/2018, I lost all of my koi due to a huge ammonia spike above 8.0ppm. I did multiple water changes to no avail, including a 100% water change. For the past 3 weeks, my filter has been up and running with no fish.

I discovered that my municipality went from chlorine to chloramine as a disinfectant for the water. The problem became worse because I used a dechlorinating product, which in actuallity stripped the chlorine and left free unbound ammonia in the water, hence the cause of the huge spike.

Since my fish died, I have scrubbed and cleaned my pond, started with one week of running the filter, loading the filter with microbes (Microlift) the next week adding the UV light and the following week adding additional water. This past week I inserted two 3 year old koi into the pond and all went well until today. Mind you I tested the water two times a day and all was good.

I returned home this eveing from work and discovered my new fish dead in the bottom of my pond. The water again tested well within the limits:

• low pH 7.0, high pH 7.2
• nitrates 0.0 ppm
• nitrites 0.0 ppm
• ammonia 0.25ppm.

The water is clear and my koi once again dead! Can anyone help me out with this issue? I am annoyed and frustrated. I had some of my koi for more than 7 years and I hate the fact that my pond remains empty and every time I try I seem to kill my fish.

Answer 1

What I think might have happened before your koi started to die is this. As the temperature in your pond got above 20 °C, the koi eggs started to mature, and after some days of temperature still rising, the koi might have spawned (releasing the eggs into the water). Koi eggs are light green, so they can be really hard to see in the water.

As these eggs are eaten/broken down, they release a large amount of organic matter. This matter breaks down to ammonia, creating the ammonia spike you had (this is what happens in my pond some years, but not always).

The fish died and you did what you could to bring the level of ammonia down by changing the water. The problem is that by doing so, you added water containing chloramines (I say chloramines, as this is a group of chemicals that is used in treating drinking water).

Here is a link to a relevant article on Wikipedia about chloramines. It contains some useful information about how to remove it from the water.

One of the problems is: the remedies we normally use to remove chlorine from the water in our ponds only removes the free chlorine, but chloramines keep on producing chlorine so it is harder to remove it. The only remedies that effectively removes chloramine are the ones containing sodium thiosulfate.

In your question I can see you register some ammonia, but no nitrite or nitrate. If you can measure ammonia, you should see some nitrite in the water after a week or so, and later some nitrate should be present in the water.

Ammonia is produced by a multitude of bacteria and some of them can tolerate some chlorine, but the rest of the bacteria in the nitrogen cycle cannot tolerate chlorine at all.

So all this ends up in getting rid of the chlorine (chloramine) and after this you need to replace the bacteria and let the pump and filter run without the UV light for a week or two, before you slowly start to add some fish.

I really hope this solves your problem. Having a garden pond is meant to be relaxing and fun and not stressful like it has been for you lately.

Some water suppliers use CO₂ to adjust the pH of the water, but this is released from the water in your pond within a few hours, so I do not think this is a factor when it comes to the death of your fish.

Answer 2

What I would do/ have done : Fill pond, run filter /aerator, no additions. After a day put in water plants ; hornwort, anancharis, etc, and any surface plants that are legal in CA. Water lettuce, salvinia and duckweed work well ( the koi will eat the duckweed as soon as they are put in). Water lillies work very well, the floating plants will shade and lower the temperature. I am in the Houston TX area so get a lot of sun. My pond is in about 65 % shade but enough sun that the lillies bloom well, my temperature is 80 F while swimming pools are 90 F. After a few days put in some inexpensive fish, swords, platies , mollies, etc. If they are OK in a few days , your water is tested and you can put in koi . The koi will not eat the small fish but the small fish will eat the mosquito larva which the koi will not eat. You may get a green water algae bloom until the floating plants out-compete the algae for nutrients ( do not use algeacide).