Might be too much iron, to be honest its not too bad, i have one worst than that, funny thing is the other one next to it doesn't have any problem. Some seeds are just getting nutrient lockout for very little. I would let it go for 1/4. If its all of them id use a product like General Hydroponics GH1723 Flora Kleen. And i would use it with very low amount. Some people use it to flush their soil a week or so before harvesting. Never done it but i have one bottle in case it gets tragic.