Light is probably the factor here. The tank light seems bright in the photo, esp for LED, and simply reducing the duration may be all that is needed. I would also get some floating plants.
I would not worry about the flow. Water flow is often mentioned as the source of problem algae, but the truth is that flow is rarely if ever the direct cause. I have found quite the opposite...algae is most often present around the filter return. I tend to look at lights and nutrients, individually and in balance, first.
You don't mention fertilizers, so I will assume none are being added. As for nitrates, that is ideal. My tanks are in the 0-5 ppm range, except the one 90g which has been at 10 ppm, and not surprisingly it has an organics/algae issue that has gone on for nearly two years now, and which I am at long last getting resolved (I hope).
I will just relate my experience with ambient daylight, since you mention the sun. This can have a much larger impact than we might tend to think. I have my 8 tanks in a dedicated fish room, and a few years ago I noticed that brush algae became troublesome; I reduced the light duration by an hour, algae went away. But it returned, and the following summer. It then occurred to me that the daylight entering the room was more intense and longer in duration during the summer. The next year, I covered the windows with blinds and heavy drapes that were kept closed the entire summer (being a dedicated fish room with nothing else in it, this was easy to achieve). No problem brush algae, and now, some four years later, I have had no algae issues in the summer. It sometimes only takes something like longer days or brighter light to overthrow the balance, and give the advantage to algae.
Byron.
