I love to do 100% water changes. For some tanks i do only 100% for others 1-50% and 1-100% a week. Sorry about Doyle. One of my betta bites it but not that bad i guess. As soon as it grow he bites it. One time he bit it when i took his plant out. I took it out because i want to change his plant. I have this tail bitter for 2.5 years though.
Good with your other bettas you have many of them.
so sorry if it columnaris it can spread really fast. I would recommend medications or 3tsp/gall of the aquariums salt as soon as possible .You can start with 2tsp/gall and then another day increase to 3 tsp/gall. And if you don't have medications start with aquarium salt and water change .
I hope he will survive :(