Off the top, I will suggest that the light is probably on the bright side. T5 fluorescent tube lighting often uses HO (high output) tubes which are roughly 1.5 times brighter than comparable T8 tubes (I have T8 still) and two tubes over a tank is a fair bit of light. Reducing the duration can sometimes help to restore the balance, but if the light intensity is significant to begin with, other steps are also necessary. Floating plants is one, so that's good here.
I am not familiar with JBL Ferrepol fertilizer, and I checked their website and they don't give a list of the nutrient ingredients but they say it contains "iron, potassium and other essential minerals and trace elements" so giving them the benefit of doubt it is probably fairly complete. If you are so inclined, when it is gone you could look into Brightwell Aquatics' FlorinMulti, which I know is a comprehensive supplement, or the very similar Seachem's Flourish Comprehensive Supplement. I use the latter, but both are basically the same. An excess of iron for example can cause algae issues, as indeed can an excess of some other nutrients too.
I don't think it is an issue here, but I would recommend you check your tap water GH with the municipal water folks just to be certain of the level. There is quite a significant difference for some plants between 3 and 6 dGH. The tap/source water is the prime source of the "hard" minerals, as most fertilizer preparations assume one has moderately hard tap water and thus these minerals are minimal. I have near-zero GH in my tap water so I have to supplement the hard minerals above what is in the liquid preparations, though only in three of my seven tanks, because of the plant species and their calcium/magnesium needs being different.
So to the brush algae...once you find the balance that works, you will still see this (or some other type of algae) but it will be under control. Plant species, the number of plants, the amount of fertilizers, water changes, fish load/feeding...all these factor into this balance.
One often reads that CO2 is a cause, but this while partly true is a bit misleading. It is the balance that is out, and CO2 is but one nutrient in that balance, but for most (without CO2 diffusion) this nutrient will be more significant because we are relying on natural CO2 and therefore cannot control it like we can the other fertilizer nutrients and the lighting. This is why one sometimes has to fiddle with the lighting duration; it is mainly the CO2 that one needs to balance with the light, and then supplement the other nutrients accordingly.
Water flow is another "cause" sometimes mentioned, but this is something of a misconception too. Water current or no water movement, neither will "cause" brush algae if the light and nutrients are balanced to begin with. I had a tank running for over a year that had no filter, no light, just a heater; it sat in the south-facing window. It was an experiment of sorts, and the plants grew fine, the fish were happy, and I never had algae issues once I got the daylight under control to balance. Yet in my larger tanks with canister filters, I do see some brush algae if the balance gets out, and there is always a bit around the filter intake and return tubes. So my point here is that the algae takes advantage of the imbalance, whether there is water movement or not. If anything, water movement would tend to be more likely to lead to algae.
Hope this helps, but feel free to question.
Byron.
