I've had a planted tank for 2 years or so. It's somewhat low light, 64 watts of T8 light over a 55 gallon with a homemade CO2 injector. I have had great growth in all my plants, with of course, periods of up and down, mostly due to my negligence. I am including a picture of a problem I have had all the time. This black spot algae, which doesn't scrape off, it always leaves marks behind. My thought is that it is green spot algae turned black. What do you think it is? What would be my best way of combating it? Thanks.