Could it be that there is too much copper either in your fertiliser or your water supply (lukewarm water from the copper hotwater cylinder)?
We tried to kill snails one day with copper sulphate and while most plants are unaffected, the vallisneria started yellowing and dying. Looked very similar to yours actually.
We rescued it by immediate water changes when we noticed (about 2 days after we put the stuff in), and although most leaves died it recovered and grew new leaves (with snails still alive!!). It was, however, an established tank with good root systems. Not sure what would have happened if they would have been newly planted.