Yes the proper treatment for columnaris is furan 2 and kanaplex used TOGETHER, I also don't know if you have a school but tetras are schooling (off topic), columnaris will wrote it your whole tank if you do not treat for it, you can move the affected fish to quarantine treat him individually and monitor the other fish, or the alternative is to euthanize the Tetra, monitor your other fish have the meds on hand in case you see anything and treat them immediately, also turn down the temperature on your tank to 74-75 degrees.