Just about all nuisance algae control comes down to controlling nitrates and phosphates. You must take out as least as much as you put in. Also, and this is very important, just because the water tests show low or zero values doesn't mean you don't have a problem. You do have a water quality problem. The nuisance algae is using up the nutrients, and growing.
There are several major sources of nitrates and phosphates. The main one is overfeeding. Most people feed way too much and way too often. The next most critical area is the source of your water used for mixing salt. Always use RO/DI water. It is worth getting a TDS meter so you can verify the quality of th RO/DI water before you start. Another possible source is the carbon being used. Also, anything that dies and isn't removed is a source too. Some low quality salts being used might be a problem, but this is rare.
The next thing to review is your filtration system. This includes live rock, sand beds, skimmers, reactors, mechanical filtration, and refugiums. Make sure the live rock and the sand bed (if used) is reasonably clean. Many use a power head to blow out the live rock from time to time. It also help to vacuum the sandbed, but do not go deeper than about 3/4 of an inch if using a deep sandbed.
Make sure you are using a quality skimmer that is large and powerful enough for the tank you have. Skimmers remove a lot of "golp" before the rest of the filtration system needs to deal with it. Many people size the skimmer far too small. Use one rated for at least double the size of your tank.
Reactors like a phosban reactor can help a lot also, but you need to make sure it's clean and working, and you'll need to replace the media from time to time.
Mechanical filtration media should be cleaned at least weekly. Remember, all a mechanical filter does it move the dirt to a single point for easy removal. It's still in the system until you clean it. Incorrectly installed refugiums can also be a problem. Make sure it's being fed with cleaned water (skimmed and mechanical filtered) so that it doesn't become a settling tank and trapping dirt.
In addition, good circulation helps a lot. Make sure the circulation is the tank is good. You may need to add power heads, or do other things.
Keep up the water changes. You'll need to do a lot of them. the 10% a week or 25% a month rule is fine for a tank with out a problem. To control algae, they will need to ba a lot larger and more often. Keep in ming that if your nitrate reading is 20 ppm, a 50% water change will only cut this to 10 ppm. To really get it down, you need to do a lot of water changes.
Lighting adjustments can help. Make sure your bulbs are reasonably new. It can help to reduce the amount of light, but unless you fix the underlying problem, you'll still have an algae problem. Remember there are systems out there that use massive amounts of light and do not have an algae problem.
Another major mistake made by many people it to attempt to control algae by introducing fish such as tang, or other livestock, expecting them to eat the algae. About all you are doing is recycling the algae. The fish eats the algae. Then it poops, putting nutrients right back into the system.
Never ever use medications, or "red slime killer" products. At best, you'll kill off one type of nuisance algae only to get it replaced with another. At worst you'll mess up the entire system. Just don't use them, and save yourself the additional problems.
It takes a lot of time and effort to control algae, but it can be done.
