Goldfish and Costia

Of all of the parasitic infections that can affect goldfish, Costia is one of the hardest to identify. It can only be seen under a microscope and – even then – can be hard to spot.

But despite its small size, Costia can have a serious effect on goldfish if left untreated.

If you are worried that your goldfish may be suffering from this infection, or you want to know the warning signs to look out for, read on to learn more.

What is Costia?

Costia is the name of a small parasite that can live on your goldfish and in your tank water.

The most common time of year for outbreaks is the springtime, though your fish may have been carrying the parasites for some months before showing symptoms.

Symptoms of a Costia infection in goldfish

The symptoms of a Costia infection are quite easy to spot once you know what you’re looking for.

Your goldfish will probably develop slimy-looking patches over their gills and head, and will clamp their fins down close to their bodies.

Goldfish will sometimes go off their food and their condition will usually deteriorate quickly.

You may see your goldfish gasping for air and rubbing against tank furniture in an attempt to ease their irritation.

What causes Costia in goldfish?

Like many common goldfish diseases, Costia parasites are usually introduced with the addition of new fish and their infected water.

Quarantining new fish before adding them to your tank is important. However, Costia may be present for several months before your goldfish show symptoms, so short-term quarantine doesn’t always help.

Costia can also survive out of water for a time. It’s therefore vital to sterilize any tank equipment that you use on multiple tanks. If you don’t, you could transfer Costia parasites from one tank to another.

How can Costia be treated?

Costia is relatively easy to treat by adding salt to your tank water.

Add one tablespoon of aquarium salt per gallon of water every twelve hours for three applications.

This will usually eradicate costia from the water and from your fish.

However, if one tablespoon doesn’t work, increase the dosage to three tablespoons per gallon. Again, perform up to three doses, each twelve hours apart, and you should find that the Costia symptoms start to disappear.

3 thoughts on “Goldfish and Costia”

    • Hi there,

      Some salt treatments work different to others, so it is best to check the box to see what it suggests.

      But typically when treating salt, we like to add a little amount each day for a week, and then perform a partial water change at the end of the 7 days.

      If the infection still doesn’t clear up, we do the same again the following week, if the fish still doesn’t improve after a month, we start looking at other causes for the problem.

      But remember that after you have treated the fish, perform regular water changes to remove the salt, as goldfish are freshwater fish, and should not be exposed to high amounts of salt for long periods of time.

      hope this helps.


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