jvelkgrove,
Very nice post when looking for advise. Very thorough and helpful to those wanting to help. Thank You...
Green Hair Algae (GHA) will grow even in low lighting. I believe red slime will as well. Your moon lighting is providing enough light for the algae to grow, even at night. The Algae seems to be getting light for 24 hours a day, based on your setup unless there is overlap. Think about the real sky at night. The moon rises and falls just like the sun, and it isn't up specifically when the sun is down, unlike Minecraft (don't ask me why I felt compelled to add that last bit). Anyways... I recommend gradually changing the amount of moonlight from 16 hours to something more like 4. You probably don't want to change it all at once, maybe trim off 4 hours each week. I will gradually reduce the moonlight until I have about 8 hours of total darkness.
Light is only half the problem though with Algae, there is still the nutrients that it is getting, which is probably the bigger problem. Algae eats Nitrates and Phosphates and Carbon Dioxide, the spores eat Ammonia...
Have you tried skimming 24 hours? If you're getting a good dark skim, then that means there is something it is removing from your tank. Just because it is removing it, doesn't mean it got it all in that 12 hour time. You might want to bump its run cycle up. It's not likely that you will skim too much. Most of the time the only reason why someone has their skimmer on a cycle like yours is if they feel it is removing trace elements needed by the corals. You can test for those and observe them to see if running it longer periods is indeed affecting them.
I have already set my skimmer to run 24 hours, I will pay close attention to trace elements on my next water testing.
Have you tested the water you are using for your ATO and your changes? Does it have any Nitrates or Phosphates in it? If so, this may be feeding the Algae with each water change and top off. I've never thought about testing the water for ATO and water changes for Nitrates/Phosphates. This will be the first think I do when I get home today, Maybe the second... A beer just might be first!
How much water flow do you have? If the flow isn't strong enough, especially at the surface, there might be too much Carbon Dioxide building up in areas. This would especially cause the Red Slime Algae to grow as it thrives on low flow. Perhaps you need to adjust your power heads or even consider adding another one or substituting a more powerful one. I have both stock return nozzles pointed at the surface, The surface water has very good movement. I have two MP10's that are in anti-sink mode via Apex, The water movement thought the tank is very good.
When you feed your fish, is everything getting consumed? How much settles on the bottom? Excess food dissolves and becomes Ammonia, which Algae spores need. Then the rest of the Ammonia cycle happens, thus introducing the food needed for the Algae to grow. Perhaps you need to consider feeding every other day, but smaller amounts instead of larger amounts every 3 days. This will give the fish a better chance of consuming all the food they need and less of it settling. Feeding, I feed only what my 3 fish will consume in about a 2-3 minute period, I know not 100% of the food is consumed, I really try to not over feed. I will keep a close eye on this...
When you do a water change, do you clean the substrate at all? Do you use a baster to blast your Live Rock at all? Stuff is settling all the time, especially if you are dealing with a possible flow problem. You'd be amazed of how much stuff has settled on the rocks if you haven't been blasting it. With every water change, it's a good idea to blast the rocks a little bit of water flow to get stuff off of them and up into the water column to be syphoned out or directed towards the sump/skimmer. You don't want to stir the sand bed up like crazy, but getting the surface cleaned off during this process helps a lot as well. Some people do it in increments, like 1/3 of the sand bed with each water change/cleaning. All the stuff that is settling (food, fish waste, invertebrate waste, etc.) is slowly dissolving into the nutrients Algae is living off of. I will be siphoning the substrate for the first time on my next water change later this week, I have always blasted the LR while doing water changes. Its amazing how much stuff the LR collects!
If this were a new tank, I'd tell you not to be too concerned and let the Algae do its thing. But since this is an older tank we are talking about, it's time to try changing things up gradually to see if you can get on top of it. In most cases, you want to make changes gradually, as completely changing something in a contained ecosystem can have adverse effects. That being said, I think it would be safe to try the 24 skimming right away, as well as the change in the feeding schedule.