How to Treat a Bacterial Pond

How to cure a bad case of bacterial pond. Nagging problems with many koi getting ulcers and other bacterial infections? How to cure it without antibiotics. 

Two things will bring on ulcer disease:  Poor water conditions and parasites
Ulcer disease displays itself in many ways.  Fin reddening, erosion, body sores, reddening and swelling of areas of the body, mushy or grey gills to name a few.  Catching them in their early stages and injecting them with antibiotics is almost always successful and is actually quite easy.” Karl Schroeler

The author then goes on with detailed description of different antibiotics and how to apply them. However, it is my suggestion that we treat the root cause first, then treat the ulcers, and if it at all possible, do it without the use of antibiotics.

Poor water conditions lead to parasites which in turn creates a break in the slime and skin barrier which can be infected by bacteria or fungi or both. Poor water conditions lead to chronic stress and a compromised immune system which favors the pathogens. So normal progress is stress condition => weakening the immune system => parasites => bacteria and soon fungi to invade on top munching away at the smorgasbord of the broken body.cells. 

A high load of organic or inert substances, dissolved or floating freely, creates conditions favorable for pathogens to thrive by providing a source of nutrition or place to colonize. The higher the organic load the better the chance of opportunistic organisms to overcome the natural immune system of the fish. Once these defenses are broken and ulcers appear, the infected fish becomes a secondary source of the bacteria to increase the load of bacteria dramatically.

Very quickly things can get out of hand, especially if the main culprit is a virulent pathogen. You scream for help and next all fish are injected with flavor-of-the-month antibiotic. It might help, if you have the appropriate antibiotic, because it helps with fighting the onslaught of foreign organisms. If you are lucky.

An ulcer is formed when a break on skin allows a free flow of water into the fish because of osmotic pressure. For a time the fish can cope with the influx of water by using natural processes to control its water balance, like the gills and kidneys.  But these are all at the expense of energy and the loss of other substances like sodium chloried (salt) for the fish. The fish gets weaker and eventually gives up fighting. It normally dies from kidney failure. Some will survive longer than others. They will swell from the uncontrolled intake of water and exhibit the typical “pop eye” or “pinecone” symptoms. When this happens, they will rarely survive.


What to do – Treating an “ulcer pond”.

There are many ways to address the problem of “bacterial pond”. The best is, never to let it happen. This method we and others have used often with success..

1.    Setup a separate or quarantine tank, sufficient in size to accommodate the whole collection. Provide for aeration and maintain good water quality. Do water exchanges if necessary.

2.    Maintain a 3 kg/1000 liter salt concentration in the temporary tank. If you do a water change, top up the salt. If you don't like sums, get a salt meter.

3.    Optionally you can add your favorite antimicrobial like Chloramine-T, BAC-50 or SteriPond to the temporary tank.

4.    Now, dip the fish, one by one in potassium permanganate solution 1 g/10 liter (fresh, non-salted water) for 7 minutes. (bowl big enough to cover the fish) Aerate well. The timing is critical. Cover the dipping bowl with a net to prevent the fish jumping out. If the fish get fidgety, rather take it out. Check the gills of each fish to make sure it will handle to 6-7 minutes, otherwise cut the time to 2-3 minutes.

5.    When 7 minutes are over, put the fish in the temporary tank.

6.    Do this with all the fish in the pond. Smaller fish can be done in groups keeping them in a net while dipping.

7.    You will notice the fish turns brown in patches, along the tail and fins, and around the ulcer wound. This is all infection organisms being attacked and killed. The necrotic tissue around the ulcer will be brown.

8.    Try not to add too much of the pond or tank water to the dipping bowl, otherwise redo the dipping bowl mixture.

9.    Finally the pond will be empty and the all the fish in the temporary tank. Do not feed.


Now turn you attention to the main pond.

1.    Dissolve and distribute around the pond potassium permanganate at 8 g/1000 liter while using the running pump to mix it.

2.    Once the pond is mixed, turned off the pump. This prevents the biological filter from getting to much of a “knock”    

3.    Once the pinkness of the potassium permanganate turns to brown manganese dioxide, replenish the potassium permanganate with 4 g/1000 liter pond water.

4.    Stir for a short while by switch the pump on

5.    IMPORTANT: keep the pinkness of the pond water for at 8-10 hours by adding more KMnO4 if needed.

6.    Overnight the brownness will (should) precipitate and then do a good backwash of the system.

7.    After the KMno4 treatment you can neutralize and clear the water with 20-30ml/1000 liter of hydrogen peroxide at 12% strength or aid the settlement with bentoniet clay like Sparkle

8.    The pond should now be receive cured fish.


Now back to the temporary pond.

1.    Repeat the dipping every second day for three days.

2.    But, after the second dip examine the fish thoroughly for signs of infection. Those with NO infection (brownness) around the fins, mouth, body and no ulcer, can be returned to the now ready main pond. Remember to peek at the gills!

3.    After the third dip the ulcer should be healed over and show no sign of infection. Make sure there in no infection. Normally after the 3rd dip all is cleared. Do not be afraid to dip the difficult cases once more.

This procedure WILL clear the pond of the “overgrowth” of bacteria, organics and other organisms. It WILL “knock” the filter, so be vigilante for ammonia and nitrite. It WILL cure ulcers unless it cannot get to the infection from a virulent bug. You can put your collection on a course of antibiotic feed, but it may only be needed in case of serious ulcers. Remember: healed ulcers are covered with a thin white film with no brownness on the perimeter of the wound. Do not touch or rub that scab. It is part of the healing presses.

Depending on how efficient the filtration of your pond, this treatment may have to be repeated once a year.


Servaas de Kock
2013-02-08 and updated later.


Pathogen: A disease causing organism, whether parasite, bacterium, fungi or virus.


Koi-PPM: ready made potassium permanganate solution that can making  treating and dipping easy.

Hydrogen peroxide to terminate action and clear the water.

Sparkle is a bentonite clay used to “mop up” organics.

Many alternatives are around.