Sudden death of my Frontosa cichlids :'(

this morning around 7, i found out one of my frontosa dead when i wanna to feed them. my cichlids community tanks is 75gallon, two frontosa, two electric blue jempsey and one white african cichlids, i have no clue why my frontosa died? can anyone help here? the water condition is good, no fighting so far peaceful, filtration setup stable too, really need some help here, beside i wanna add on something, i had another green terror keeping in the same tank with them but seperate them using glass, so they are 6 fish in the tank.