To be precise, costia is a microscopic protozoan flagellate. In other words, like a lot of common goldfish illnesses it is caused by a parasite. Costia usually attacks goldfish that are already ill by destroying skin cells.
It is important to treat costia immediately as it is one of the goldfish illnesses that will prove fatal within days.
The symptoms of costia:
- Clamped fins
- Rubbing against objects in the aquarium/pond
- Awkward swimming
- A more recognizable and definitive way to tell costia from other goldfish illnesses is redness under the goldfish's chin and along the back.
- Sometimes by looking closely a whitish film can be seen over the affected areas.
Treatment for costia:
Raising the water temperature and/or salt dips can prove effective against this goldfish illness. Remedies are also available from pet shops.