I would say maybe switch to a less potent antibiotic, or even medicated food if you can find it. Using tetracycline is really hit or miss. Some people find it kills their bacteria, others find it inactive. Kanacyn (aka kanamyicn) is also useful agains this bacteria, and should be a little more gentle on your filter.
Here is some more info on treatment. I am not the author of this.
Treatment:
Ensure that your water conditions (e.g., Ammonia, Nitrite, pH, Nitrate levels, and water temp.) are within their proper ranges. If not, perform a water change and/or treat the water accordingly. Recommended medications include: Furanace, Fungus Eliminator, Fungus Cure, Furacyn, Furan-2, Triple Sulfa, E.M. Tablets, Tetracycline, or Potassium Permanganate. Medicated foods are also recommended. Columnaris can be highly infectious and may quickly kill all aquarium inhabitants; therefore, early treatment is essential. All fish, including those not yet showing visible symptoms, as well as the aquarium they inhabit should be treated.