Betta fins reddening

Question
Hello,
I love keeping tropical fish and keep many aquariums and fishbowls with bettas.
Three of my 11 bettas in bowls, have developed
a thin red line along the edge of their fins.
I treated with Melafix and it went away.
I have one beautiful pearly-pinkish male betta
that still has a little of this strange red at the tip of his tail.
What is this illness and how can I prevent it?
There are no other symptoms of illness on the bettas.
I change their water every three days.

Thank you for your time and help,
Sincerely,
Susie

Answer
Susie-
I'm very sorry but I don't think I can answer your question. I've never heard of this problem before. It seems like you take very good care of your bettas, so I would just say to continue with what you are doing. You might try asking another expert for help too. Best of Luck!
Emily