First off, that is brush algae (the "bushy" algae on the plants, etc). This is probably the most common problematical algae we find, and it can appear differently. I'm not sure if there are separate species, or if conditions cause this, but most sources identify it as "black brush algae." Technically, it is a red algae, though it tends to be dark green (to black sometimes), very dark brown, or dark gray. Keeping it in check is "simple," (yes, everyone laugh...:rofl
once one understands the cause. And this is simply finding the balance between light (intensity and duration factor into this) and available nutrients. If this balance is suited to the plants, and no more, you will never have trouble with this (or most other) algae. It will be present, but under control. This is the only algae I have dealt with the most, over the past 20 years, and I always win in the end.:clap:
It is said that the true Siamese Algae Eater will deal with this algae, but I never recommend a fish to deal with this sort of problem. First, this fish has needs; it should be in a small group, it gets 6 inches in length, and this makes an impact of its own on the biology. There are several fish very close in appearance, and some of them bring worse problems, plus they won't eat this algae anyway. It is better to deal with the cause of the algae.
You have fairly bright light with T5 HO tubes. The green spot algae is another sign of this, btw. If you could remove one of the three tubes and have two light, you would be in a better position to get this under control. I would sometimes go even further in reduction, but as you are using diffused CO2 the two tubes should be sufficient. I would stay with the 6700K, but if you prefer the cooler white then one 6700K and one higher kelvin (12000K). I will just say that I did have a bit more trouble with this algae when I had this mix, so that might be something to consider. The high blue light is less useful to plants, but algae are not so fussy. You are down to five hours duration, and shouldn't go lower, so reducing the intensity is the only way here. You might be able to increase the duration.
I would not use Excel. First, it is a liquid "carbon" supplement that should not be necessary nor used with diffused CO2. Second, it is a highly toxic chemical, glutaraldehyde, that is dangerous to begin with, notwithstanding anything Seachem may say to the contrary. I won't pursue this now, but can if asked; I have written on this in other threads a while back. Ironically, some say this Excel will kill this algae; obviously it isn't doing so here.
The other nutrients I can only comment on generally. You have CO2 which puts you at a higher balance level than my experience which is with low-tech natural systems. But I can say that increasing any of these products beyond what the plants can utilize fully, can cause this algae. I saw this in my 90g; twice weekly doses of Flourish Comprehensive brought this algae on with a vengeance; reducing to once got rid of it within a couple weeks. I repeated the experiment with the same results, so there is no doubt that the additional nutrients could not be used by the plants, and algae had the advantage. My only advice here would be to cut back on the fertilizers and after a couple weeks observe the results. I would certainly stop dosing phosphorus. There is sufficient phosphates in fish foods for any aquarium plants, and there is some evidence that phosphates do encourage this algae.
I would also highly recommend floating plants. Not only will this provide some shade which will be appreciated by the fish, but the algae will be affected too. And with a reduction of fertilizers, this might more readily achieve that balance.
Last comment, on starting over. I wouldn't do this yet, as the same thing is likely to occur. As for somehow sterilizing stuff, that won't stop the algae. Algae spores are in the air, and they invade this or that aquarium for reasons not understood. I have 7 tanks running in a fish room, and this algae has never appeared in three of them; I've no idea why, as conditions are much the same in all tanks respecting light, plants, fertilizers, fish. Rhonda Wilson used to say the same in her monthly TFH column. You mentioned this tank was fine...for how long before this started? And do you recall any changes you made to lighting or fertilizers or whatever about that time?
Hope this is of some help. Don't hesitate to ask further.
Byron.