We have very hard well water and a water softener. I have never had a problem with my seedlings - until last spring.
At first I blamed the potting medium (which was different from previous years). My plants were stunted and yellow and just didn't want to grow. I transplanted them into a different potting mix, but that didn't help.
One day, I looked at the soil through a magnifying glass and spotted SALT CRYSTALS!!!! Then I knew what my problem was and I managed to salvage my seedlings by flushing them and watering with rain water.
We decided that too much salt in the water probably wasn't good for our health either, and we finally installed a Reverse Osmosis system
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Why hadn't I had problems in previous years? Because I always saved any leftover tea and coffee for watering, thinking that there were nutrients that the plants would appreciate. We made our tea and coffee using bottled water because it tastes better! Last spring was particularly cold, so I used warm water from the tap for the seedlings......
Linda