My red betta (Ariel) is now suffering from minor fin rot after noticing holes earlier on

. I guess those holes turned into fin rot? anyhow Ariel has been and is again in quarantine. His top and bottom fins has shed some and its a pretty mess at the moment. Besides all these his behavior is normal, active as ever etc. Im still thinking back how could this happen to one out of three bettas? water change etc are all the same for each and my other two bettas are in good shape.
Any other suggestions anyone would like to share in order to cure Ariel asap pls? all suggestions will be appreciated, thank you.