What kind of water do you use for top off water? What kind of freshwater do you use to mix salt in?
How much do you feed, exactly? What kind of fish do you have? Explain your filtration system to us? How old are your light bulbs? How much internal flow do you have in your tank, not including your return pump, skimmer pump, filter pumps etc. Just flow from your powerheads, and how are they positioned?
You will read zero for nitrates and phosphates in your system because the hair algae has sucked it all up into it's tissues. I highly suggest that you use an algae scrubber to rid your tank of it. I think you could also look into getting your magnesium level up with Kent Marine Tech-M. If you can steadily keep your magnesium elevated at 1600ppm for two weeks or more, you will see the algae disappear. However, once it's gone, the nitrate and phosphate will leach back into your system, so you need to be prepared to do a large water change and get your skimmer going very efficiently. You can also use that algae scrubber on your system to keep algae growing on the screen, rather then your tank, when magnesium levels come down from 1600ppm to 1400ppm, the algae will start growing again.
Here's a few basic husbandry steps:
1. Lower your lights to 1/4th of the time that you usually have them on. Put your lights on a timer, and keep the schedule for two weeks or more until the problem dissipates.
2. Feed less, rinse your food before feeding, and target feed your corals when possible.
3. Pick it out of your tank with your fingers every time you do a water change.
4. Only use RO/DI water with a TDS reading of 0 for top offs
5. Only use RO/DI water to mix your salt in for water changes.
6. Reduce phosphates and nitrates by protein skimming and the use of macroalgaes
7. Wash your bioballs occasionally in old saltwater
8. Replace your filter media more often
9. Run carbon in the system
10. Buy and use a phosphate reactor
11. Buy a clean up crew that will eat your hair algae. Limpets, lettuce nudibranchs, Mexican turbo's, lawnmower blennies all do a great job ridding your tank of algae.
12. Do scheduled water changes
13. Replace your lights when they are due to be changed, before their color spectrum shifts.
There are a few other things, but you get the general jist of it. All it takes is some hard work, money, and dedication to the hobby.
