Hi !!! Glad to hear that your rescued fish is getting better. So maybe the best idea is to forget about MB and medication in general. Just keep on doing what you have done already: clean water and lots of love for your new fish friend [and maybe the occasional salt].
Hi !!!! Did you had the chance to solve the health issue with your new betta. I was a little worried since I don't know what happened. Hope it all turns out well. Sometimes it just takes time and effort but in the end some health issues are solved for good. Keep on going my friend !!!