weak growth

Can someone please help me out? My plant just started getting like this out of nowhere. It was doing well and then one day I checked and it looked like this. I noticed baby fungus gnats all in the soil so I put yellow sticky fly traps in the soil and it is catching as much fungus gnats as it can...

Photo actually makes plants look better than they do in reality. I had no other light in my grow room other than my 600w HPS which was masking the yellowing of leaves until the situation had become very obvious. Things were going perfectly up untill 1 week ago. I believe this is a case of...