Problem with Your Koi

Here’s a problem that one of the readers of my e-book, “What Your Pet Shop Owner Won’t Tell You About Keeping Koi” encountered (sent

via email):

“Chris,

I have recently tried to rear 6 - 8 week old koi fry in a nursery pond which is set in the wooden decking surrounding my house and they have all died in the space of one month. The fish show no signs of tissue damage, they simply stopped eating. I can’t get to the bottom of the disaster but I was wondering if it was possible that the decking, which overhangs the pond, could have leached out some toxic preservative?

Have you ever come across this happening?”

Here’s my answer to him:

“If your koi pond is new, it is unseasoned and a new pond needs to be seasoned (or ‘cured’) before introducing the fish.  And if there are too many Koi, the ammonia and nitrite build up very quickly.  One of the results of a build up of these excessive toxins is the fish not eating.

So if I were you, I would

1.  check all the parameters of the water using a test kit (level of ammonia, nitrite, nitrate, chlorine, chloramine, aeration, temperature etc).  If your tank is new there usually is not enough colonies of bacteria to get rid of the ammonia and nitrite (as you probably know, ammonia is a by-product of fish waste/urine and leftover food).  It takes about 4-6 weeks for the bacteria to grow to sufficient numbers to control the levels of ammonia and nitrites.

The ammonia that is produced is changed by different types of bacteria into nitrites and then into nitrates.  This cycle lasts until the amount of bacteria reaches a level that can sufficiently eliminate ammonia that is produced.  You should constantly (I recommend daily) check the levels of ammonia and nitrite in the water.  If the levels of either ammonia or nitrite show a positive reading, the pond is still cycling.  A seasoned pond should show zero levels of ammonia and nitrite.

2.  If you had lost your entire collection and are starting again, scrub and clean your pond and fill it anew with fresh water.  To avoid a repeat of what happened, you can try the Fishless Cycling method.  This is where you generate the cycling process in a pond without any koi.  In this method, you introduce pure ammonia into your water in a controlled manner to spur the growth of bacteria.  The aim is to replicate the production of ammonia as though you had koi in the pond, thereby causing the growth of bacteria colonies.  You can read more about how to do this in Curing a New Koi Pond.

Only when the bacteria levels have grown to sufficiently eliminate the ammonia and nitrite, do you introduce koi into the pond, but do so very slowly (one koi per week).

A New Koi Pond...cure it

3.  If you still have some koi left, make sure they are not overcrowded so as to prevent a build up of too much ammonia too quickly.  If overcrowded, you should give some away.  Assuming they are not overcrowded, check the levels of all those substances I mentioned (especially ammonia and nitrite).  If you get a positive reading, the thing to do is to make a major water change then place some fresh activated carbon in your filter.

Then you need to help the remaining koi through the cycling process.  Do so by making frequent partial water changes.  This prevents excessive build up of ammonia and nitrite.  As you change water, constantly evaluate the ammonia and nitrite levels using test kits.  You need to change water frequently (everyday if necessary).  One thing you can do during this process is to buy material that breeds the bacteria from the pet shop, like some filter media from an already established tank.  Adding this into the water speeds up the growth of bacteria.  Remember to get rid of the chlorine and chloramine from the tap water you use when you change water.  Use a dechlorinator or another type of water conditioner.

So the frequent water changes and activated carbon is to effectively reduce the levels of ammonia and nitrite which are toxic to your koi.

4.  If you have already done the above and the levels of ammonia, nitrite, chlorine, chloramine and any other toxins are zero, then the problem may be some toxins leached from your wood decking.  The wood may be treated with preservatives to make it last longer, prevent termite infestations etc.

Yes, these chemicals may leach into your water somehow.  If so, then I would add activated carbon into my filter to try to absorb it.

Personally, I think it’s more likely a build up of excessive toxins in the water that killed your koi”.

If you are facing a problem like this one or if you have a new koi pond (or planning for one), do learn from this reader’s experience.  Solve problems like these and get more tips with “What Your Pet Shop Owner Won’t Tell You About Keeping Koi.”