Not to hijack this thread, and I hope you don't really mind but I am having the same problem and feel it would be pointless to make another thread about the same topic. So if it is okay, I'd like to post my information as well.
Its a 20 gallon tank standard with 30 watts of lighting(flourescent) and I use Flourish weekly, also. One of my plants, (my 1st live plant to be exact) has green leaves plagued with black spots like it's dying or something. It has no name since I bought it named as "Assorted Plant". The lights are on for about 10-12 hours. I also have a bubble bar w/ airstone to increase aeration. Tank temperature is around 74-78 degrees Fahrenheit. Plenty of filtration and I do weekly water changes (I'm no beginner to the fish world at all). So what could be the problem?