After planting and cycling my 40 gal, I added seven Black Skirt Tetras. After two weeks, I added seven Bleeding Hearts. Two weeks later added six corys. All perameters keep solid and it appeared to be a peaceful community. After a couple more weeks - today all the Bleeding Hearts are gone! The Black Skirts and Corys are fine. Tested water and every thing ok. What killed my Bleeding Hearts?