Help :Seedlings yellow leaves

my seedlings are about a week old and the leaves are turning yellow they are in a rockwool cube and i am just wondering some type of remedy to fix this

you need to find out what is wrong and fix it.
temps? PH to high/low?, have you used ferts?(to soon) to much/little water?

yeah mrmadcow is right...that happened to me a while ago....it was overwatering

I think it maybe over watering because one of my seedlings leaves are curling downward as well how often should i water the cube and how much water

Check your humidity! This same thing happened to me and it was because of low humidity. If everything else is in range, (PH, water schedule, light, etc) then it's most likely a humidity thing. Get a dome or cover with a zip lock bag with corners cut off.