tomato problem

Does any one know whether nutrient deficient tomato plants can be salvaged or is the damage permanent?  I am growing my toms in pots under cover and they just don't look right to me. They leaves have a dull dry look,  and they are. Also they have a purplish tinge and although they are fruiting they are not growing strongly. They have stopped trying to send out sideshoots and generally look sad. Perhaps I was slow to start feeding them. They are getting half strength tomorite about twice a week. Should I up it and could I give them some seaweed plant tonic as well, or is that not a good idea?