oh yeah, tap water @ 400ppm is some rough stuff.
The plants don't absorb all of that calcium carbonate stuff in the tap water and it is a natural, slightly alkaline additive. Cannabis plants don't like the calcium carbonate b/c it will literally begin to raise the soil pH, throw off the organisms which depend on a slightly acid condition and block absorption of N-P-K. Think of it like this.. Get a large glass bowl and fill it with tap water, after it dries, look at the film of white crap stuck to the glass bowl. That shit is like chalk on your roots..
R.O. filters are great! They usually cost (at my last shopping) about $150 or so and 5-10 gallons a day on a new system is no problem. - Make sure you have good water pressure at your house. I choose a slightly more hippie approach. I have rain gutters which collect the rain water from the roof and it fills a swimming pool.. lol.. I know it sounds funny but let me assure you, the water is fk`ing perfect!!! My rez is 100% organic teas and I use rain water to fill it. The pH NEVER moves... ever!
..Citric acid and some ele sulfur.
make sure you give them time to recover, don't add more water until it dries the medium a little. Regarding the epsom salt comment, I do not see anything I would assume could be resolved with epsom salts (magnesium sulfate).
I would assume your soil has been staying too wet with Tap water, and flushings, ect.. The roots are not taking. I like using cior/perlite. I fertilize with every watering of rain water, ect..