I use this product and I love it! You can use it everyday but I would only add a cup a day. I don't follow directions. I use it about every other day and I dump in a lot. The only thing a lot of it does is it turns your water a little cloudy until the corals take it in. I don't know where to buy it online, I get it at my LFS.
I bought mine from ***************.com. They sell it for $20 per gallon. I have no idea if it helps or not but its green and smells funny so it has to do something.