Yellow leaves

Please help what can this be my friend is growing hydroponics this what he sent me

Needs feed everyday

That is really low in the basket, you are going to be getting light into your rez which will cause root rot. any info on the type of water you are using and the light ?
Do you have roots coming out of the bottom of the basket yet ? Are the buckets black?
Looks like magnesium def. but without any other info its hard to help your buddy.