Cotton Wool Disease, aptly named for the cloud-like white fuzz that appears on the bodies of fish, manifests with distinct symptoms. In its very early stages, goldfish may appear itchy, rubbing against the substrate or tank walls. Within about three days, initial symptoms emerge as small, white granules (clusters of mucus) begin to form. These can easily be mistaken for other illnesses such as Ich or Velvet Disease due to their appearance.
As the disease progresses to the middle stage, a white film develops around these initial granules, spreading over the body like a cloud. At this point, the fish may exhibit reduced appetite and more lethargic swimming, often staying at the bottom or floating listlessly at the surface.
In the final stages, the disease spreads across the entire body, secreting enough mucus to obscure the scales, severely weakening the fish as it stops eating. If the mucus accumulates near the gills, it can cause respiratory distress, leading to further debilitation and eventual death.
The early and mid-stage symptoms of Cotton Wool Disease can easily be mistaken for Ich or Velvet Disease due to their similarities.
To distinguish between these conditions, consider the following characteristics unique to Cotton Wool Disease:
- Fish rubbing against objects with no visible abnormalities on the body.
- White granules appearing within a few days.
- Granules primarily forming on the body with irregular shapes.
- White masses that have a raised, bump-like texture.
Using these features as a guide, you can compare and differentiate Cotton Wool Disease from Ich and Velvet Disease.
Ich manifests as tiny white spots (0.5 to 1mm in size) on your goldfish, with each dot being nearly the same size.
Velvet Disease, also known as Costia, features white dots on the tail fin and body caused by Epistylis. While the spots are also circular, they are slightly larger and more raised compared to those of Ich.
Cotton Wool Disease, or Ichthyobodo, does not form hemispherical bulges like Ich or Velvet but rather irregularly shaped white granules. These granules appear not as neatly aligned dots but as clumpy, uneven white masses on the body. They resemble more of a crusty debris than distinct granules.
Even with these comparisons, distinguishing these diseases can be challenging, but hopefully, this information will be of some help to you.
Causes of Cotton Wool Disease
The cause of Cotton Wool Disease is parasitism by the flagellate Costia (Ichthyobodo) or the ciliate Chilodonella.
Costia (Ichthyobodo) is about 0.01mm in size, making it impossible to see with the naked eye. It’s also difficult to detect even under a microscope, so the diagnosis of Cotton Wool Disease is often based on symptoms. This parasite is active in temperatures ranging from 3°C to 30°C, becoming most active around 25°C. However, its reproduction tends to weaken in water temperatures above 28°C.
Changes in water during water changes or sudden temperature shifts due to seasonal changes, particularly temperature drops, can lead to poor health in fish, making them more susceptible to Costia infestation.
On the other hand, Chilodonella, measuring between 0.05mm and 0.09mm, cannot be seen with the naked eye but can be identified under a microscope by its oval shape. Like Costia, Chilodonella thrives in a wide range of temperatures but is particularly active below 20°C and becomes extremely active and reproduces through division below 10°C (conversely, its activity decreases above 28°C).
Similar to Costia, Chilodonella is more likely to infest when sudden temperature changes disrupt the health of the host.
Regardless of which parasite is responsible, Cotton Wool Disease is most commonly caused by sudden drops in temperature or poor health conditions.
From experience, it’s often observed that fish purchased from stores and introduced into new aquariums develop the disease, likely due to the stress of transport and environmental changes weakening them.
During seasons of lower water temperatures when Costia and Chilodonella become more active, the best preventative measures are to properly adjust the water temperature and treat with methylene blue.
Cotton Wool Disease can occur on its own, but it can also coincide with other diseases such as Gyrodactylus, Trichodina, and fungal infections.
In cases where these diseases occur simultaneously, even if the symptoms of Cotton Wool Disease improve with medicated baths, you might still see recurring signs like rubbing, white granules, or fuzz.
Since different medications are effective for each condition, it is appropriate to move to the next treatment as each set of symptoms resolves.
Effective Medications
Both Costia and Chilodonella can be treated with medications. Trichlorfon is not as strong as formalin but is still a potent medicine, so you must use it in the correct amount.
Effectiveness of Salt Baths:
Salt baths have been proven effective for eradicating Costia, but a high concentration, specifically 5% salt solution, is necessary. According to a report published in August 2003 by the Salmon and Trout Resource Management Center News, authored by Shigehiko Urawa, a 10-minute bath in 5% salt solution resulted in a 0% survival rate for Costia, demonstrating the effectiveness of salt in eradication. However, 5% saline is very strong; experimental data showed that the mortality rate for salmon fry was about 20% after a 10-minute salt bath and approximately 90% after 15 minutes, indicating a high risk.
While a 5% salt bath can eradicate parasites, the effectiveness of a 0.5% salt bath remains unknown.
Many people use dyes, but some also use oxolinic acid (KanaPara D, Green F Gold Liquid) or furan and sulfonamide treatments (Erbaj Ace, Green F Gold Granules). Personally, I believe that oxolinic acid and furan and sulfonamide treatments, being primarily for bacterial infections, might not be effective against these parasites, although some people use them, perhaps to treat or prevent co-occurring diseases like fungal infections.
Since the cause is parasitic, dyes or eradication agents are generally effective. However, there seems to be no definitive answer for the best treatment for Cotton Wool Disease.
If you are unsure which medication to use for treatment, it’s best to start with less toxic options like methylene blue or malachite green, which are safer for goldfish. If these prove ineffective, then consider switching to stronger medications like oxolinic acid, furan treatments, sulfonamides, or trichlorfon.