Fish Health - Costia

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Also known as Ichthyobodo Necatrix

Red fish - CostiaHave you ever wondered what costia looks like, and what it can do to your fish? Well here it is. Just one un-quarantined fish was put into a full system of what were a class fish, and now half the stock is dead.

I was called in when the fish had that many chemicals, toxemia was killing them as well.

You must do microscope scrapes of more than one fish and look for the red patches or white-ish patches on what seem unaffected fish. And scrape those areas.

Use a saline mount for one of your scrapes. Make two mounts one with saline and one without. Then check the saline one (keep cool don’t put cover glass on mount until the time has passed and do not put under scope for 45 mins).

If the costia is still alive you have Asian salt resistant costia. If the costia is dead then its European form which responds well to salt.

If you have bypasses on your filter, use fresh malacite and formerlin for Asian form. Then half water change and redose in 7 days. But with salt leave for 20 days.

Which ever method you choose, scrape fish everyr 3 days. Never under estimate this baby or it will kill all your fish then wait around for new stock.

infected fin infected fin infected fin

-----Original Message-----
From: Cindy
Sent: 15 May
To: Fish Helpline
Subject: Fish Helpline
Fields and Data
Test_Kit_Date: 09/07
Ammonia : 0.0
Nitrate :
Nitrite : 0.0
pH : 7.0
kH : 60
Water Temp: C: 68
Pond_Size : 4500
Drug : none yet
Symptom-38: White Waxy Hard Droplets in Fins and Tail
Enquiry : Photos being sent separatly. this condition appeared quick fast. This fish was shown at a major show on 2 months ago and was perfect. I am certain she was fine 3 weeks ago. Now, her body is clean and clear, no veining, no heavy slime coat, but ALL of her fins are thick, lumpy, and very red. She also has low levels of costia, although other fish in her pond were also scraped and scoped and no parasites were found. Any ideas?
this concerns me greatly....most of my show fish are in her pond and I am now so worried she may have transmitted this to them. I have had this fish for about 9 months. A fish from the same breeder, aquired at the same time is normal.
thanks for your help.


-----Original Message-----
From: fishhelpline
To: cindy
Subject: RE: Fish Helpline
Date: 15 May

Hi Cindy
any other fish in that show have problems now? was fish ok before show? how long after show did this start? Check all your other fish by night with a torch so there fins stand out ive seen this before its not good news


-----Original Message-----
From: cindy
Sent: 15 May
To: Fishhelpline
Subject: RE: Fish Helpline

Lawrence, thanks for your reply. The show was in early march, so about 2 months ago. This problem started within the last 2 weeks. I looked at the other fish yesterday and did not see any problems with any of the others, but I will inspect them more closely this evening after dark just to be sure. I do not think any others are showing this problem. What do you think it is? You have an idea? I do plan to send a sample of the tissue to a lab for analysis, but that will take some time.
thanks again.


-----Original Message-----
From: Fishhelpline
Sent: 15 May
To: 'cindy'
Subject: RE: Fish Helpline

hi Cindy
turn round in lab should be within 5 days. there is a school of thought which i agree with. costia has aggravated the harpies gene (pox) within in the fish. opening the main feeder blood vessels supplying the fin muscles very similar to a nail on your hand having a infected quick so infecting them then to the finer vassals and allowing aeromonas a foot hold this keeps the wound open some say its not aeromonas but columnaris which is feasible as columnaris dissolves tissue as it feeds and allowing costia to feed protected by the pox mass if it infects the ventral fins costia will follow up fishes vent into lower intestine results are self explanatory. now that what your pictures look like but pictures and the real thing are poles apart. hope this helps. note there is no such thing as low leval costia you have costia or you don't costia does not move in levels all that's keeping it at bay is fishes health once that changes it will go into binary fission mode and rip through your fish like a balloon in a holly bush



The following article is acknowledged with thanks from Doc Johnson of

CostiaBefore Costia kicks everyone's rear-ends, please read the following. There are a lot of Japanese style shows going on, and the aftermath of several over the last twenty four months has read the same: Fish are mixed, returned home, novices do not take precautions, Costia kicks their butt so bad it looks like a "virus" until it's accurately diagnosed and cleared.

The significance of Costia

Life was simple for a fish vet in the early nineties. Japanese fish were too expensive for the average hobbyist and their beautiful wiles had not been fully discovered. The usual pond-call involved a poor filtration system, high ammonia levels, a sagging pH and a case of "garden variety" Trichodina on some hardy domestic fish.

One would simply recommend a treatment of salt coupled with an upgrade to the filtration type and flow. A week later, all the fish are recovered. Doctor Johnson is a hero.

Now, partly because my exposure is more widespread and Japanese fish are so much more prevalent, the fish veterinarian sees much more challenging things.

Salt resistance has arisen in the following parasites:

Flukes - 20% of cases are completely resistant to salt treatments at any level. Most cases are cleared at zero-point-nine-percent but less than thirty percent still clear off at the old zero-point- three-percent standby level.

Trichodina - 20% of Trichodina cases do not clear at even zero-point-six-percent. There are rare cases where trichodina does not clear even at zero-point-nine-percent which is also stressful to the fish. Formalin or potassium permanganate are recommended.

Chilodonella and Ich have never been recorded as being salt resistant in the least bit.

Costia has shown some resistance and it is part of the reason for this article.

Some outbreaks of Costia have been known to be salt resistant for at least two years. I saw my first case of salt resistant Costia in late 1996 on some Japanese imports straight from Los Angeles California. The Costia resisted zero-point-nine-percent stiff salt solutions and finally required Formalin for clearance, which works extremely well.

Costia is responsible for a lot of the recent fish mortalities we've seen after Japanese style shows.
There are several reasons for this, which I wanted to discuss here.

  1. Costia resists drying. Empty tanks, nets, even dried nets and hoses, can transmit infective Costia from one group of fish to another. Folks are not very cautious about nets and bowls. A retail facility that does not rinse or disinfect their nets and bowls between tanks is capable of infecting every fish they sell, whether they originate in infected tanks or not.
  2. Costia is not always evenly distributed among fish populations or even upon an individual fish. What I am conveying is that you may have a collection of fish in which only a small percentage of fish will have Costia. Numerous biopsies of some healthy looking fish are negative while the affected fish swarm with Costia. An immune capability is suspected among those fish that do not show morbidity under Costial attack. Secondly, the Costia may exist in small "patches" on skin and gill and be missed in routine scrapes. It is imperative that whenever you're biopsying a fish, you biopsy general locations such as between the gill covers and the pectoral fins. But that you also biopsy any red or white patches upon the fish.
  3. Costia is easily missed under routine microscopy. There is an important reason why. For a definitive diagnosis, a microscope should be able to perform a competent 200X scanning power. 400X is sometimes important to diagnose Costia when numbers of organisms are low. Many of the new Chinese microscopes are labelled with 40X optics and 10X oculars but despite the listed calibre, are nevertheless poor at imaging at that power. So you often see nothing.
  4. Leaving the iris diaphragm open on your microscope defeats the necessary contrast to diagnose Costia. You should use the lowest power light and the smallest aperture on your iris diaphragm. For more on this, refer to the book or the web page for a diagram of these important microscope parts.

It has unfolded in several scenarios that after shows, participants begin to lose fish, sometimes with ferocious rapidity. The hobbyist does a few biopsies and does not find an organism. Rumours of a virus begin to spread. Finally, a competent biopsy is taken and viewed under a powerful microscope and the Costia is discovered. Unfortunately, it is often too late for a bunch of fish, when the proper biopsy and microscopy is done.

Recently, a good friend of mine had some fish that were beginning to isolate. The fish developed reddish lesions on the skin, and face. Some white slime was attendant at these lesions. Biopsies were taken and nothing was found. I asked if he'd biopsied the specific lesions themselves, and indeed he had not, because he felt it might further traumatize damaged tissues. This was a logical concern, to be sure, but as I stated before. Costia can exist focally, in small patches that can be missed unless they are deliberately scraped.

Formalin is the best treatment for salt resistant Costia.

The general idea of this Formalin treatment is to apply fifty PPM Formalin (two millilitres (equivalent to cc) per ten gallons) to a tank with the filter bypassed. Increase circulation with a floom or with a spraybar. Run this level of Formalin for two hours and then execute a forty to fifty percent water change, with de-chlorinator. Repeat the treatment in 72 hours. Do this Formalin treatment for a total of three treatments and you can rid your system of Costia and Flukes.

There is an important, auxiliary treatment for Costia which I have found to be very effective at slowing down the infection. I have been using topical hydrogen peroxide 3% USP applied directly to the patches. Here's how that works:

This has been particularly effective in some of the Ranchu goldfish that I keep, where facial or body scarring might be highly undesirable. These fish are able to handle high levels of salt with aplomb, so I usually endeavour to treat the focal lesions with peroxide. Then I salt the system to zero-point-six-percent to-zero-point-nine-percent.

There are several important follow-up points to this article.

During and after your treatment of Costia with salt at zero-point-three-percent, it is imperative that you serially biopsy your specimens to make sure the numbers of Costial organisms are decreasing or are absent. If you're still seeing Costial organisms after 72 hours in zero-point-three-percent salt, some level of resistance can be assumed. At that point you should either increase the salt concentration to zero-point-six-percent to-zero-point-nine-percent or consider Formalin. You should not use Formalin at fifty PPM with the higher levels of salt. I have used zero-point-three-percent salt and left it in during Formalin treatments. This has caused no problems in cooler water with high circulatory rates. I caution you that stiff (zero-point-nine-percent) salt solutions coupled with Formalin might create an oxygen availability/transfer problem. All oxygen tension problems become more acute under conditions of warm water, e.g. water over eighty DGF

A theory exists that Costia may exist in the fishes' cloaca, using these sequestered surfaces as it might use any other extension of the surface of the fish. In the cloaca, the organisms may be protected from short-term treatments like Formalin, and this ability to inhabit the cloaca is proposed as one of the possible causes of unexplained recurrence of the disease. Currently, the time has not been available to biopsy the cloacas of infected fish nor to attempt or ascertain the safety of clearance of said pathogens by swabbing the vestibule with peroxide.

Do not let your fish out from under your effective treatment until all biopsies are negative for at least three days.


  1. Costia may be salt resistant
  2. Costia may be focal, not general in distribution and so it is often missed on biopsy.
  3. One should biopsy "patches" and lesions, not just healthy tissues.
  4. Salt and Formalin are mainstays to treatment.
  5. Peroxide (three percent USP) swabs may speed healing at the lesions.
  6. Biopsies are IMPERATIVE during treatment to establish sensitivity to the chosen treatment and clearance
  7. A very good microscope and competent technique are mandatory for the diagnosis of Costia as it is easily missed.

Thank you for your time.

Doc Johnson