700 ppm is a bit high for seedlings.
The pH needs to be as close to 5.8 as possible, especially in dwc with seedlings that aren’t relying on cotyledons anymore.
As was mentioned, water temp is important in DWC. Ideally you should keep it under about 72 degrees Fahrenheit. With plenty of aeration and good microbes like from hydroguard being added can help, and dwc can be done with enough airflow and aeration up to nearly 78, maybe almost 80 degrees, but again you need the good microbes and the dissolved oxygen levels need to be as high as possible with at least high airflow aeration.
Also make sure your water levels aren’t too high – contributing to "drowning the roots at the base of the plant, some air space is good below the net pot, maybe a good centimeter at least, even up to a inch or more with well developed roots. And the bubbles should throw enough moisture up to the hydroton to keep the seedlings happy with roots that long.