I used to add the odd Earthworm into my South American Cichlid Tank.........A Firemouth had a Battle for almost an hour over the Juicy worm, with a Convict attached at the other end.... One would pull one way, the other back the other way.
An entertaining 'Tug Of Worm' ensued ........just wish I'd owned a Video Camera back then!
............I also had a small Angel who was so greedy, he battled all the older ones out the way when the Bloodworm was added......unfortunately he ate too much and I believe this was his downfall.
+ My mate had his 6inch Plec snapped by a Blue Crayfish lobster thing, on the night I warned him about them!.......he also lost 2 Silver Sharks the next day.... Luckily he was exchanged back to LFS before the rest were devoured.