Hi! First post here! I've been dealing with cyanobacteria over the last month for the first time in 20+ years of fish keeping and I'd like to share my experience. This is the first time I've started keeping a planted tank, and it occurred, I think, due to my lack of experience with plant needs. I think what some of the people are saying about it occurring for any of a number of reasons, or most likely, a combination is probably correct.
My situation occurred about 3 weeks after planting. I have a 100 gal 6' long that has been established for over 2 years with a sump/trickle filter, diy moving bed filter, and diy internal canister filter with powerhead. Fish had begun dying for (mostly) unknown reasons. I had started ignoring them mostly but I think there was a water issue too. After deciding to switch my setup, I added a pretty sufficient amount of plants and started adding soil under the substrate. I upped the light wattage from ~80 watts total to ~190. I also removed the trickle filter and replaced it with a diy internal canister and removed the other internal canister and attached powerhead. Subsequently, the water current was lowered from a spray bar and power head to a single return from the new internal canister (a 500 gal/hr pond pump previously used for the sump). While the return may actually be greater due to the reduced head, it is more directed than the spray bar and not sufficient to move the water along the whole 6' footprint. Fish load was extremely low, 3 fish, a red eyed tetra, a silver hatchetfish, and a Chinese algae eater. The former two make almost no waste, but there is a sufficient amount of algae in the aquarium to keep the CAE plump and he creates a great deal of waste. Trying to go the whole "low tech" route, I read Diana Walsted's book, and she suggests allowing a mulm buildup to occur to keep the soil charged with nutrients. I also was really lazy about the light, didn't have a timer, so the lights were on anywhere from 14-18 hrs a day. I was also dosing with Leaf Zone at 1/2 the recommended dosage twice weekly.
In all, I had higher and long light levels, no water movement, increased nutrients, a low nitrate (0), and a buildup of organic matter. Incidentally, the BGA started showing up en masse, along with a huge outbreak of hydra. Two weeks later, I have no visible hydra, and no further BGA. First, I broke up all the visible BGA by hand. Then, I vacuumed the surface of the substrate as well as any rocks and driftwood to remove most of the mulm and BGA buildup. This amounted to about a 10% water change. I reinstalled my powerhead with a sponge filter and used the air input to allow the water to be oxygenated. I then did a three day blackout, followed by two days of 12-hour light, and then two days of 6 hrs on, 2 hrs off, 6 hrs on light. I then stirred up the substrate again, revaccumed, and increased my plant load by adding a 15" Marble Queen sword with a rather dense root system and a moderately dense carpet of chain sword. Since then, about 2 weeks, I have been religious about an 11 hr daily photo length (I'll get a timer soon). I quit dosing the water column but have continued to add soil to the substrate little by little. No BGA since the blackout.
It also occurred in another aquarium at the same time. This one was a new set up. 30 gal low tech. 1.5" MGOPS with 1" gravel/sand cap. 60 watts light with same photo period as the 100 gal. This tank has one adult angelfish and a single green cory. The gravel is dark in this aquarium as opposed to the play sand substrate in the 100. BGA showed up first on my group of hornwart and spread to the substrate, rocks and driftwood, including roots of Java fern I have attached to the wood. The plant load in this tank was much lighter, though the hornwart was such a nutrient hog (grew 6" in a week) I didn't think I needed it. I followed the same procedure as the 100 gal, except I threw out all the hornwart, replaced it with water sprite, and didn't clean the BGA off the driftwood or rocks. The high growth rate of the hornwart caused shading off of the lower parts of the plant and there was a severe build up of needles that I hadn't noticed until I removed it. I removed all the needles. I redirected the canister filter output which had previously sprayed across the back wall to now spray across the length of the aquarium. After the blackout period was over, the BGA had turned black. After the lights were turned back on, some of it greened up, but has not spread in two weeks. I'm simply watching it. I don't mind algae in my aquariums, I think it looks nice, but I want to make sure it doesn't spread as it was. As far as I can see, all spreading has stopped and it is isolated to a few spots along the edges of some rock and two spots on the driftwood.
In both cases, only minimal water was changed and no antibacterials were used. The main issues appeared to be eliminating organic buildup (mulm), decreasing photo period, and increasing water flow. No increases in nitrates occurred, no new fish have been added. Nitrates still read 0. Soon enough here, I'll be adding fish to both aquariums and I'm sure that will seal the deal.
I hope my experience helps a bit!