Deficiency or Toxicity

it goes like this: toxicity leads to deficiency. (deficiency leads to symptoms. symptoms lead to suffering.)

you overfeeding your plants has been locking out nutes on them. too much of one element (usually N) hinders the plant's ability to take others (such as p/k, cal and mag). This can also result from pH off, but in this case, 1200ppm does indeed sound too high. never go by the manufacturer's instructions. I give my plants 7 weeks in flowering a ppm of 800-1000. your plants can make do with 600-800 I think. If you're on a nute regimen that works, don't up it just because the table says so. wait for your plant to give you a very slow indication of N def (uniform yellowing of bottom leaves), then up it slowly. Unless you really know what you're doing, and know your strain very well.

it's better to use RO water (but know that it usually requires an addition of mag), because it gives you more room to put balanced nutes in the ppm that your plant can afford. but your problem was not the water. You probably have some salt residue in your medium, so flush it well, then go on feeding as usual, and even go lower on the ppm (down to 500-600), just for a while so it can bounce off. if it will be hungry you will start seeing different symptoms and will be able to up it carefully again.

It is important that you filter your water to get rid of chlorine and chloromine. You can't bubble chloromine out. They both kill the micro life in your plants and then the plants are not able to uptake nutrients.

Get a water filter first. There are lots of inexpensive ones out there. Then reduce your nutes significantly. I fert according to height. If a plant is two feet tall I give it 200 ppms if it is 3 feet 300 ppms. This is when it is first starting in flowering. As the weeks go on I am upping the ppms by 100. I maybe get to 800 when I am near harvesting. I do a water, fert, water, fert regimen on my plants. I use Roots Organic Soil.

Never believe the dosages on the bottles even on AN. I have a large garden and use only AN and never go by their charts. Every plant is an individual and when they are not hardy too much fert will kill them.

I believe you have a nutrient lockout because of your hard water and you keep giving them more fert because the plants show signs like deficiencies. But the real answer is the lockout causing the deficiency which is ultimately caused by your water. 250 ppm on tap water is very high. You should be concerned and filter your home water before drinking.