I have a fish mystery on my hands

First topic message reminder :

In the last couple of days i found one have eating cichild i seen five so far. I'm baffle since the the largest fish in the tank....... I have no clue whats going on in there..there a list of fish in my 55g on the left hand side under my avatar..i need help what do i do? I don't wanna keep on loosing fish this way..horible way to die if your ask me.

The plants are diying from the top down also have green hair algea witch i never had last time i tested the water Ph was at 7.0 hardness 5 nitrate 0.5 nitrite0.1 amonia 0 and c02 at 15 ppm.

You've dying plants and algae growth. How old is the light bulb? Old bulb contributes to algae due to lost of spectrum.