With the information given...
Rockwool
Distilled water
Superthrive
...

They were getting too much nitrogen, enough to scorch them, but you've flushed, which was a good call...
Distilled water has incorrect mineral content for plants unless you're running calmag. Switch to tap water unless you KNOW your tap water sucks, and if so, you're going to need an EC meter and a bottle of calmag.
Rockwool needs to be soaked in a solution at pH 6 until it holds that pH stable... the factory sends the shit out at a pH too high for pot. You need to adjust it.
Water with tap water adjusted to pH 6-6.3. Are they going into dirt or dro? Dirt, leave the tops of the cubes EXPOSED so they dry fully between waterings and don't rot the stems, and as the roots grow out, you'll be letting the pH drift back to about 6.8 and leave it there. Dro, also make sure there's a drying cycle if you're running flood n drain, and keep the pH stable at 5.8-6ish. Don't forget that the babies need a minimum level of minerals.. calmag... ftw...