I've killed 3 bettas so far feeding beefheart(fed over 200 bettas with it). I think it choked them to their death. Immediately after they ate it, a minute later they started swimming like crazy! and laid on their side. 30 minutes later, dead. I don't know if it was the beef heart, beef heart not trimmed well from the fat, or suffocation. Make sure to grind well if you use it. And use it at your own risk. :)
P.S. I use it because it's affordable and stated to increase betta growth by other betta hobbiest who use it. I don't know if it's good for them or not. And yes, it's the heart from a cow.