Ahhhh, Jayy! I wish I could be of some help to you - best of luck, hon. Hopefully it helps the lone female diamond tetra - but like Byron said, if not - hopefully it at least STOPS with her. Please keep us posted on how things go over there. *crosses fingers*
Olympia, I'm a total fish newbie here, you know this. . . but from what I've been taught, it's really
important to actually take the time and watch your fish eat. . . to know if they actually are eating, for one - but it also gives you the best opportunity to see even the most reclusive fish out in the open. While they eat, look them over, check their breathing, their fins, scales, eyes, and gills - if you know what is normal for them, you'll realize sooner when something is amiss, and have a far better chance of getting the help we need to fix it before it's too late. This is especially important for beginners like us! Maybe you can try turning the tank lights off, but leaving the room lights on so that you can actually see them while they eat? My ADF used to be shy, but he's gotten used to eating at the same time every day - in the evening when he's most active - and now lights don't bother him a bit!
I don't know anything about betta and breeding, but I do know that. . . to me? Dogs are easier! Dogs are MAMMALS! I totally get dogs - fish? That's a whole 'nother thing that I know NOTHING
about ;) Good luck with your skinny boy. . .