Hmm....you're right. I forgot they say 1 bucket is for 55g. It probably wouldn't be enough for a 75g.
I personally would buy some dry ferts. I buy mine from here:
http://www.bestaquariumregulator.com/ferts.html. I suggest getting KN03 (for nitrates), K2SO4 (for potassium), and then using either CSM+B for micro nutrients or buying Aquarium Plants trace mix or getting some regular Flourish.
He takes paypal payment. You will have to email him your order.