Less is always more with seedlings.. Nute burn usually only hits the tips of the newest leaves first.. causing pretty much instant death of the very tips of the leaves and then spreading inwards.... Full yellowing of the leaves like that is usually a deficiency... the newer leaves on those plants appear to be more green than the lower ones.. which leads me to believe its nutrient deficiency and not burn.. Your plant food should have 3 numbers on it seperated by dashes.. I believe ALL plant foods have this number system.. The first number is Nitrogen content.. Second is Phosphorus, third is Potassium. You should be able to tell which has the most nitrogen.. by the first number being higher than the others... and your Ph should be between 5.8 and 6.3 for soil.. 6.5 is too high!