"Stringy" isn't a word I'd used to describe Cyanobacteria, it would a word I'd use when describing numerous strains of Algae, Beard,Hair,Silk,and Stag algaes to name a few. In my tanks Cyanobacteria as black, it grows on large river rocks and orniments and has yet to become invassive as most of my fish will graze on it including my Jags, fancey plecos, and cats. Supirulina is a form of Cyanobacteria. This bacteria is not always black could be red or blue-green and I view it as benificial. This stuff was/is the building blocks of life billions of years ago. Algae is a whole other story, it's probably best to have these things ID'ed before throwing meds into the water. With no pics posted for proper ID this whole thread is a guess and treating a guess with meds is just plain reckless. There are tons of sites including this one that visually will help you ID your issues, once that has been done you can correct it, other than that it's all BS.