[Assault0137]

yesterday, i found that my killifish's back has exposed, ripped white tissue all over. today, it is upside down near the back of the tank, but still breathing. this is 3 days after 2 of my 3 angelfish died of similar symptoms(white exposed stuff)(i think my dominant angelfish may have pecked them all) BUT, i have never seen my angelfish mess with my killifish... i know its not likely but is there a disease that does that?
but i digress... my main point is that i don't know if i should let it die on its own or euthanize it. my mom thinks i should just let it die, but i dont want it to suffer.