Hiya BB. Sorry to hear about your betta. Have you checked the water params lately. If not it might be worth just checking that ammonia, nitrite and nitrate are all as they should be (o-o-<40).
Have you changed the brand of food or anything like that? I suppose there could be something wrong with him - have you checked for growths, lumps, bumps etc?
As for the BN, what size tank have you got? The feeding regime sounds ok to me.
TROPICAL TANK-Elite Tank - 207litres
6 Harlequin Rasboras, 5 Emerald Eye Rasboras, 10 Cherry Barbs, 2 Bristlenose Plecs, 2 Panda Corys. 4 Dwarf Chain Loaches.
ViaAqua Tank AR620 97litres
1 red male betta, 1 BN