What did I do?

Kay I did something to my Süßwassertang, what I'm not sure. It happened about a week ago, when I was really busy with work. I've always dosed EI ferts erratically, with out having and problems. My CO2 ran out for a couple days, but then again that has never cause any problems except algae grows faster. I dosed melafix for 3 days on this tank, I've never done this before. My first guess was it was the melafix that did this, but I don't know why it would. Anyway for a short period of time my Süßwassertang grew badly, the new growth was very dark. Then it went back to growing like normal. The dark growth has since died away. The whole tank was effected. I'm kinda upset about this, cuz it's going to take awhile for it to cover up all this bad growth.