Unsure what this is... Mutant?

Hey guys,

I've recently started my 2nd grow and some seedlings are doing fine but some of them have no other growth than the cotyledons, which are starting to show brown marks and growth has completely stopped. I did notice as soon as they popped through the coco that some of them looked peculiar. 3 of the pics are of the seedlings that seem to have a problem, where as the other pic is of the one that seems to be doing great!

Growing them under a 300w CFL 6400k
temps 75-78
tap water ppm 330
PH 5.8
RH 40%

All of the seedlings are the same age around 12 days old

Im fairly new here so if i've missed out information bare with me!

Any help much appreciated!

Thanks!

some seed batches are a bad cross. Ive had some of my ak-48 crosses do the same thing.... 1 out of every 5 actually grows into a good plant. it all depends on genetics and who bred them. I would start some more to make sure your going to have 3-5 good plants in the end to work with. 12 days loss is alot better than a 3 month loss. good luck!!

This is what i did think, Luckily i have a few different types and some spare seeds to get cracking! Thanks for the input bro!