High,
This is caused by nuteburn. It is better to start with lightmix. I do always use BioBizz lightmix this contains enough nutrients for 2 weeks. So you can control the nutrients after 2 weeks. The Allmix contains nutrients for the first 6 weeks so you can not feed them as you wish for the first 6 weeks. I would just keep giving plain water till they are 6 weeks old and then start to build up the nutrients.
Happy Growing! 😋