seedling problems ?!

I have 4 seedlings in rockwool starter cubes. 2 of them are working on their set of three leaves but all of the sudden today the leaves are a slight yellow green and the seed leaves are fully yellow. I water 3 times a day because the rockwool cubes dry out fast. maybe i am overwatering but idk because i heard that you are not ever supposed to let the rockwool dry out. maybe they need nutrients now because it has been about 2 weeks . idk please help because you guys know how important growing is. btw this is my 2nd grow but 1st hydroponic one. thanks

start on the nutes 1/4 strength may want to put the cubes in a container to hold a little bit of mositure