Don't know where to look next, Help?

[oldfishman]

I have a 28 gallon AIO aquarium that has been running 10 months. About 6 to 8 weeks ago I began to be have green hair algae take over. I was told it was probably Phosphate and GFO would help. I purchased an API Master Reef test kit to check for Phosphate. The following are all of my measurements:
TDS top off water 0
Salinity 1.024
Ammonia NH3 0
Nitrate NO3 0
Nitrate NO2 0
PH 0
KH 8 or 143.2
Calcium CA2 420
Phosphate PO4 0

I have taken ever measurement twice today and was most surprised by the Phosphate measurement which was actually twice today and twice yesterday.
The LED lights are on 7 hours per day and the yank is not in a window nor does it get any direct sunlight. I change 5 gallons of water with new RO/DI saltwater every 7 to 10 days. I thought all my measurements were right on so where do I look next?

Also, I can add GFO or Reef Flux but I am not sure which I should try first. I posted in the new to salt water section as others my have the same problem. My wife hates the green hair algae almost as much as I do.

 

[DLHDesign]

It's likely that the GHA (green hair algae) is eating up all the nutrients you are trying to measure before you can measure them.

How much are you feeding and how often?
What's in the tank in terms of fish and clean-up crew (snails and crabs and such)?

It's likely that your best course of action will be to reduce your feeding (volume and/or frequency) and to manually remove as much of the GHA as you can. Might need to add some more clean-up-crew (possibly fish as well/instead) to help combat the smaller pieces that you won't be able to manually remove.

 

[ShaunRobinson]

Your Po4 readings will be false. You obviously have Phosphate in your tank or the algae would not be growing. It is likely locked up in your rockwork and algae. A decent Phosphate remover in a reactor if you have one will help as well continuous manual removal of as much as you can. If you are able to, when you do a water change take the rocks out and scrub them with a hard nail brush in the old water. I’m aftaid getting rid of gha is a battle but with the right approach and patience you can defeat it.