i have 75gal tank that i am using for an experimental no-tech tank. i am doing a lot of things that people say is wrong. my tank sits in front of a double-window and gets direct sun the 1st half of the day and indirect the rest. there is no heater, filter, pump, or water movement of any kind. there is a small 15w light on one half that i no longer use. there are lots of different species of aquatic plants as well as bog plants and house plants as other emergent plants.
when i first started the tank, it got a HUGE case of cyanobacteria. it carpeted the entire floor, almost every plant, and grew up the bottom 1/4 of the glass on the tank, as well as large spots in other higher areas. on the back of the tank, it grew so much that the sunlight barely got in. a couple members in here have attributed this onset to my using "bacteria in a bottle" stuff to help cycle the tank and i believe they were probably right. it stayed around for quite some time and stifled everything. nothing eats it when it is healthy, but i think some creatures like snails might munch on it when it is dying. instead of fooling with it or blacking out, or peroxiding, or using medicine, i just used patience and let the plants catch up to the nutrient consumption.
i did do one thing to help the plants out that probably everyone here has already done. i added a layer of topsoil under the gravel. it caused a big mess and i thought that maybe the lack of sunlight getting through the tank would at least diminish some of it on the front of the tank. no dice. it was just as strong as ever. this was about a month into the tank being set up. all the fish and snails lived through it and once i rerooted some of the plants and did some water changes to get the water clear again, things changed. the cyano started dying and the plants starting growing. it has been a week now, and it is completely gone, despite having ample sunlight, no water flow, no fertilizers (topsoil had no ferts added), no CO2, and no treatments.
everyone can draw their own conclusions from this, and the findings might not seem to help someone with an established tank that gets an onset of BGA. but there it is. the addition of fresh soil destroyed the BGA completely. btw, when cyanobacteria is waning in health, it will begin to look spidery and loose, and it will produce little bubbles that cling to it.
whatever your conclusions, i thought the algae forum was in much need of a positive post