Originally Posted by MatPat
I've used a toothbrush also, but I didn't brush it on concrete first. I'm gonna have to try the concrete idea!
I can't believe how oblivious I am: instead of shortening the bristles on concrete, you can simply take a pair of scissors and cut the bristles to size. Ha ha ha.
Most toothbrushes have long, pliable, and uneven bristles -- not very useful attributes for scrubbing hard things off of a flat surface. By shortening and evening-out the bristles, you are making the bristles less pliable and much denser, which translate to improved scouring power. Perfect for those stubborn green spot algae, among other types of algae.
For heavy infestations or on large tanks, it may make more sense to invest in products recommended by Jan, trenac, and MatPat. I am much too cheap to throw away my trustworthy free