I've been running a previously owned 75 gal for 1.5 years with similar lighting, a "GLO" 36 inch dual T5HO with one actinic and one 10,000K. The fixture is 2 inches above the water surface. A few months after setting up, a mushroom started growing.... my FOWLR turning into a mixed reef all on its own. Unfortunately, it was subsequently consumed. I think one of my pencil urchins was the culprit. Late last year a single zoanthid polyp began growing on a piece of LR near the middle of the tank. It has since grown into a colony of more than a dozen polyps. I keep the colony fairly high in the tank and provide supplemental feedings of mysis etc. Another piece of LR spontaneously sprouted some Spindle Weed (Neomeris sp.) as well as Grape Caulerpa. Yet another piece of LR saw a form of red macro algae take hold, Halymenia if I'm not mistaken.
I've also been successful in keeping a BTA hosting two Maroon Clowns under this set up. The anemone did real well until a few months ago when it decided to wander down and behind the rock work where I was unable to feed it properly or keep an eye on it. I let it be for some time, hoping it would move back under the light on its own. It did not and I became concerned since the tentacles were starting to bleach from lack of light. So I bit the bullet and rather than try to pry the BTA loose, I did a major LR renovation to get it back under the lights. The thing was half coloured and half bleached by this time. So far so good, the BTA has stayed put and made a fine recovery, good colour and starting to grow again. Not sure why it moved down in the first place, but I think it's safe to say that the current lighting is sufficient. All this to say/illustrate that you should be fine with respect to lighting on your 75 gal FOWLR.
I do plan to upgrade my current lighting in anticipation of setting up a 90 gallon tank. I've dismantled a used MH/VHO lighting fixture to cannibalize for parts and plan to use the two Workhorse 8 ballasts to run four 48 inch T5HO lamps with individual reflectors. I'd also like to get the 250 watt MHs back into service but don't want to use the old magnetic ballasts the fixture came with, they're monstrous and probably inefficient. I plan to run them on separate electronic ballasts, one for each bulb so as to control them individually.
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