Cyanobacteria is caused by excess organics. Without organics, cyano is not going to appear, regardless of light, water flow, etc. I have 7 tanks running, four of which have very little water movement as they are filtered with a single sponge filter, and they range from 10g, 20g, 29g and a 3-foot 33g. If minimal or low water movement were responsible, these tanks would be covered in cyano.
I have had cyano in one tank, twice. This was my former 70g setup, which had a canister filter the same as it does now, and the same as my 90g and 115g. Again, water movement has nothing to do with cyano. Nor does light, though too much light, in the presence of organics, can trigger it. And too few water changes, overstocking, over-feeding...these can contribute too.
To make sure I understand things correctly, mikey: the light is 4 24-inch T5 NO tubes, you have CO2 diffusion, and a partial soil/Eco-complete substrate. How long is the light period daily? And I assume the CO2 is not left running during darkness (= tank lights off)?
Also, could you post a photo, just in case this is not cyano but is more normal as another member suggested. And a photo of the tank to see the plant level would help.