help tips of seedlings leaves brown

ive 3 seedlings bout 1" - 1" 1/2 in rockwool feeding them ph 5.5 water only they are bout 10 days or so old. only 2 leaves on them but the tips are turning brown on the leaves should i be feeding them neuts yet or not. still only feeding 5.5 water. any ideas whats the prob?

the charts for ph range is very helpful, 5-8 is usually good