yes tap is that bad. Think about salts and chloramine or chlorine buiding up in you coco coir. Everything seems good with the stronger plants for a time. Now you might make it 8 or 10 weeks of flower but most likely the strong ones end up like the first weak ones. Hard to say why when everything you put in it was good. What's left ? the water.
Just start using distilled if you can't afford R/O filter. It is cheap compared to what you get in the end. Since you just flipped and a 600w you will do well. And no guessing what in it.

OHH, your smoke will taste much cleaner with no chlorine. Cause when you flush with chlorine and salts your going to taste it in the end product. You'll thank yourself.