Hair algae is one of the hardest to eliminate in my experience. Algae is primarily cause by nutrient deficiency of some sort. That can be not enough light or CO2 or insufficient levels of any one macro or micro nutrient. Also algae being primarily a single cell organism its nutrient needs to live and reproduce are very small. For example I store RO water in a bucket until I need to do a water change. While it is in there I use a pump to pass the water through a DI filter. The TDS of the water averages 1ppm. So despite having basically nothing in the water this bucket has algae. Not much but it is there. Now if I put a floating plant in the bucket (salvinia minima) is just sits there, It doesn't grow but it doesn't die either. It just sits there.
So in short changing lighting period or CO2 levels flow and filtration is may not going to do much. I think lighting and CO2 will only have a noticeable effect if you are currently very close to water parameter that algae doesn't like. And in general if you make conditions hard for algae to grow you also make the conditions very hard for plants to grow.
could my light be exhausting the co2 in my tank?
That is a possibility but in my experience with CO2 exhaustion, My PH when up. When I first set up my tank I didn't use CO2 and water PH was normally 7. When the plants were growing well my PH actually climbed to 9. And then at night the PH would drop back down to 7. Now I am using CO2 I can still see the same think. If my CO2 is sufficient my ph drops. IF it is not sufficient it goes up. If I have just enough CO2 my PH stays constant. I am currently running the tank with a PH drop of 0.2 ( I am not aiming for high light and CO2) and at least some of the plants are pearling and all are growing.
When I first noticed the PH increase when I wasn't using CO2 I simply solved the problem by reducing light level. This slowed plant growth a little but it also allowed CO2 levels from fish and air to get into balance with the consumption by the plants. Also It should be noted that my fish were never harmed by the high PH. From their behavior it appeared they didn't notice it. Also it should be noted that I have no idea if all tanks show a PH increase with CO2 depletion. So it is possible for your water to have a stable PH and still have insufficient co2 levels.
I'm highly against using excel as I've killed fish before with very small dosages
Excell is an organic acid that reacts strongly with certain molecules that are present in living tissue. This fact gives it it's antiseptic effect. However there are other types of organic acids that don't have this antiseptic quality. vinegar (acidic acid), citric acid, and gluconate are all organic acids that are commonly used in fertilizers. You could try these instead of excel. as long as you don't add enough to crash your PH, Your fish probably wouldn't notice a 0.5 PH drop due to the addition of a less reactive organic acid. This might help to insure your plants are never short on carbon.
The last thing I can think of to address a carbon shortage (if you have one) would be to add more fish. They convert food to carbon dioxide and organic waist which can help.
However with all that said I don't think CO2 and lighting will solve your problem. When I looked back at your fertilizer posts I saw a lot of talk about NPK and iron. Those are only 4 out of the 14 fertilizer nutrient plants need to grow. So in the end I think you are going to have to look into your fertilizer more. But with a focus on your micros.
I think your problem is something running out periodically in your tank. For example if you don't have enough copper your plants might be fine for most of the week but near the end of the week it runs out and your plant growth slows for stops for a couple of days. until your next water change and fertilizer dose adds more copper.. That 1 to 2 day pause in plant growth is all that algae needs in order to grow. Remember algae can grow in very low nutrient levels While there might not be enough copper for your plants the algae will have enough.
During the iron discussion on your fertilizer thread did you notice your copper dose was 0.0003ppm and your zinc dose was 0.0014ppm. Why so low? I am currently at 0.01ppm for copper and 0.02ppm for zinc. In my tank I use DI which has basically nothing in it. With regular fertilizers I couldn't get any reliable growth. I then started to make my own micro fertilizer and now I see evidence of growth every day in all my plants.
Zinc, copper, and calcium levels are frequently kept low in fertilizer since they are plentiful in tap water. The copper comes form copper pipes. Galvanized steel and brass pipes also contain zinc. Calcium often comes from limestone. Manufactures of fertilizer keep the Cu, Zn, and Ca levels low to prevent over dosing the water column. For example my tap water ( which I don't use in my aquarium) has a EPA safe copper levels of 0.05ppm. That is over 100 times more copper than in your fertilizer. So it is entirely possible you have more or less copper and zinc than I do simply because you have no way of knowing how much is in your tap water.
From your fertilizer thread it is also apparent that you are aware of the CSM+B toxicity issue. Many people having problems with CSM+B thought the EDTA in CSM was the cause. But there is a growing amount of evidence that suggests it wasn't a toxicity issue at all. But instead a deficiency issue. While CSM is a good fertilizer it is intended for use on farms, not aquariums. The mix of nutrients may be balanced for soil but not aquarium. There is a growing number of people that are now mixing there own micro fertilizer and getting good results. The fact is is that many fertilizer manufactures are simply guessing on how much of any ingredient to add put in a fertilizer. So when you compare nutrient levels in fertilizers you often see a wide variety of levels. So while a fertilizer may work well for some it might not work well for others. Simply because each has different tap water chemistry.
In your fertilizer thread you we considering reducing your iron since you also dose FE DTPA (due to your water PH). And most of that was because your CSM+B also has iron. I would however recommend you do not reduce your CSM+B levels. I fact I think your should try more csm. Or you can try what one other person has done which is to add additional zinc to your fertilizer.
Just for your reference I mix my own micro and these are my dose levels:
Fe 0.1ppm (DTPA)
I won't say my tank is algae free but it is much better than it was. The hair algae that once covered everything is no a very minor issue. Currently intermittent green water is a bigger issue and I have hard green spot algae issue that is not due to low phosphate. Know I can solve the green water with filer pads and an UV system and more scrubbing removes the spot algae but would like to avoid that.