i just went through the same thing in my 750g. Almost lost both of my aros and a big cichla. I did a ton of research on fighting columnaris and eventually went with triple sulfa. It fights gram negative bacteria and it didn't harm my rays in any way. I followed the dosage and recommended water change on the bottle along with adding salt everyday. I kept my temp between 82-84. All the fish are doing fine and eating again. One aro has a few remnants on his scales and lost his barbells. Its been a long couple weeks though. Good luck