Mitch, you PM'd me and asked me to comment in this thread, and here I am. First off, let me say that Mikaila has given very good advice, but this is not the course I would suggest. The algae in the photos is some form of brush/beard algae, and I have dealt with it since the mid 1990's. I have it well under control, and I know exactly what causes it to increase in my situation.
And the answer is, light that is higher in intensity and/or duration that what balances the available nutrients. Mikaila and I agree this far.
But I would bet that your CO2 is no where near sufficient to balance this light, and increasing the other nutrients in my view will not help but may make it even worse.
First, on the light in general. I had this algae increasing fairly rapidly, and I took the advice of some of my colleagues on plant forums that the light was the issue, so I reduced the duration in increments. I knew with my moderate light (two T8 48-inch "daylight" tubes over each of my 70g, 90g and 115g (5 foot length) tanks the intensity was not the issue, but duration. Once I got the duration in balance with the CO2, which turned out to be 8 hours, end of algae increasing. I used CO2 as the nutrient involved because I was adding everything else via Flourish and adjusting that had no effect. To further prove the light point, I then found that every summer the brush algae would increase. I concluded that the additional brighter and longer daylight entering the room was doing this, so for the past 3 summers I kept the windows heavily draped. No increase in algae. This has been steady now for 3 years.
To the matter of increasing the nutrients without adding CO2. Since late last year, I have been experimenting with nutrients to solve some specific plant issues. Something interesting happened in one tank that I think is directly related. I was dosing Flourish Comprehensive twice weekly in all 7 tanks [this certainly helped to solve some of the plant issues]. I suddenly had this brush algae increasing in the 90g, to the point that within just a couple weeks, it covered every sword leaf. I cut the Flourish back to once weekly. Within 3 weeks, problem over; all new leaves were algae-free, and now some 3 weeks later, still are. This tank gets once weekly dose, the others get twice.
CO2 is the only nutrient I do not add in any form, but rely solely on natural. I am quite convinced that whenever I have algae problems, it is because the CO2 is not sufficient to balance the other nutrients and light. Light should be the limiting factor to plant growth; otherwise algae is a given.
I believe you have double the light necessary. Two T5 HO tubes is equivalent to three T8 tubes of the same length and spectrum. I know that if I added a third tube even over my 5-foot 115g tank, I would have a real issue with brush algae, because there is no where near sufficient CO2 to match.
Duration can play into this too, but if the initial intensity is greater than the nutrient availability, duration is not going to have much impact in solving algae. You could go down to 6 hours daily, but I would expect algae still to be an issue. Heavy floating plants will help. But if you increase the organic nutrients, cyanobacteria on the floating plants might result. I had that in one tank.
Hope this helps. By the way, it would be better if you re-sized the photos before posting so everything remains within the screen frame.
Byron.
