The "tufts" you are referring to are commonly known as BBA or black brush
algae. It's a tenacious bitch It is possible to defeat it though, you
need to start adding co2. and you have too many gold fish in a 20 gallon
;o
Google bba and read all you can, theres loads of info on how to avoid it.

Damn British Bankers Association. if only the RSPCA had the financial muscle
to deal with them.