Posted by Chris Lewis ·
Columnaris moves fast, so it's worth knowing the signs before you need them.
What it looks like
- A pale or white patch, often across the back like a saddle ("saddleback")
- Fuzzy or cottony white edges on the mouth or fins, sometimes mistaken for fungus
- Fins that fray quickly, or a mouth that looks eaten away
- Fish that go from normal to very sick in one to three days
It's a bacterial infection, not a fungus, and it spreads faster in warm water. Read more on the columnaris glossary page.
What to do right away
- Test your water. Columnaris usually takes hold when fish are stressed: a new tank, a big move, ammonia or nitrite, overcrowding. Put your numbers into Water Check and fix anything it flags first.
- Lower the temperature a few degrees into the low-to-mid 70s °F if your fish tolerate it. Heat helps the bacteria.
- Remove carbon from the filter so medication stays in the water.
- Treat with an antibacterial made for gram-negative bacteria. Many keepers pair a kanamycin product with a nitrofurazone product. Follow the label exactly and finish the full course.
- Move badly affected fish to a hospital tank if you can. It limits spread and uses less medication.
About snails, plants and your cycle. Antibacterials can knock back the good bacteria in a newly cycled tank. Test ammonia daily during treatment and do a water change if it rises. Most plants are fine; check the label for invertebrates.
If you just moved an established tank into a bigger one: that move is one of the most common triggers. The new tank may not be fully cycled even with old plants and decor in it. Feed lightly and keep testing until the numbers have been steady for a couple of weeks.