LOL, before I even post what I am about to post, I can tell we have some very different opinions here. If you aren't confused by now, you will be by the time I'm done with you...haha
Based on what you have stated, here’s what I would do, for whatever it’s worth. This is what has worked successfully for me. If you’ve seen my tank, I don’t have any GHA.
First, turn off the GFO. You need some PO4. Mine routinely runs anywhere from .08 to .15 on the Hanna. My NO3 also runs about 10. Therefore, I think it’s more like Reefaholic suggests, in that your tank is out of balance. This will help get it going that way. As long as your PO4 doesn’t surpass .10-.15 or so, I wouldn’t use any GFO. Some people just can’t let go of the idea that nutrients should be zero. You NEED nutrients.
I think you are feeding ok. I wouldn’t cut back just yet. Don’t punish your fish, find better ways to balance and control nutrients. I would however feed twice daily if that’s not how you are doing it now. One cube in the morning and one in the evening, not two at once. My tank is about 4x yours in volume and I feed the equivalent of about 20 cubes a day. My fish are obscenely fat. Therefore, you could feed significantly more than you are now and get away with it, as long as you learn how to control your nutrients.
Your alkalinity/calcium/magnesium are out of balance. A good balanced ratio is more like alk 8.5 dKh/cal 420/ mag 1350. If you raise your alk to 8.5, that should push your calcium down to about 420. They work inversely of one another, not independently. If you raise alk while keeping calcium dosing the same, your calcium will fall. Same thing happens when your alk gets low, your calcium will rise. Some might see the calcium rising and cut back on dosing calcium when they should really be raising the alk.
Stop dosing magnesium for a while, assuming you are, and let it come down naturally, testing it weekly. It will likely take a couple weeks to decline to 1350 on its own. Corals don’t use much magnesium. Once there, figure your depletion rate and dose less. Your mag shouldn't be that high.
Don’t leave nori in the tank for the tangs. Tangs will pick on the rocks all day consuming micro algae and possibly your GHA if they are hungry. Leaving nori for them in there all day makes them lazy. Feed some in the evening for them and then take out what they don’t eat.
If you haven’t seeded your tank with pods in a while, get some and do that. Pods are like little ants, scurrying around on your rocks at night consuming detritus.
Don’t cut back on your carbon dosing. You are only dosing .05 cc daily. That’s a little low in my opinion. After you turn off your GFO and your PO4 starts to rise to something detectable, then you can play with your carbon dosing. If your PO4 starts to get too high (above .15), then up your VSV to bring it back down. If you don’t know how or why that works, read up on it. I think running the GFO is messing up your NO3/PO4 balance. You are adding nitrifying bacteria and feeding them with the VSV carbon (all good) but they can’t do their job without a little PO4, which you are stealing from them with your GFO. Stop it.
Lastly, your coral feeding. Every 3 days is ok, as long as you target feed and don’t broadcast feed. I would target feed them after the lights go out with a baster with the pumps off. Even once a week is fine for feeding, but if you should cut back on any food at all, it should be the coral food. Corals also consume nutrients in the water like fish poop, so feeding your fish more is also feeding your corals. Corals can also make food through photosynthesis, fish can not.
I think you said you need a new bulb on your UV. While that is good for things like parasite control and bacteria control (like red slime), it won't do much for nutrient control.
If I think of anything else, I'll add to this.
Have a little issue could use some help , the picture is tank currently as of about 10 min. ago , everything is doing well but I have a patch of GHA growing on my overflow , and one small patch on a rock and now everyday I have a very thin film of algae on my glass , never had that before , could go two to three days without cleaning glass , my UV bulb is due replacement , and dosing Carbon VSV daily .05 ML and Biodigest weekly , 15 gallon water change every other week using reef crystals always have , tank is 90 gallons with sump probably total water volume of 100 to 110 gallons. Parameters are as follows , just tested tonight.
