leaves are curling/flopping down?

hi guys,

I have a problem with my plants, the leafs are flopping down like there too heavy to hold up. There in a 205 nft tank on half nutes and ph and temp are all ok so really need sum advice.
Just look at my thread for first time grow using nft from seed for pics ....

Any advice would be great as i dont want my babies to die lol

Cheers guys

All i say is a lie and i know nothing about anything!