This will depend on how much PO4 are going into the tank vs. what is being removed.
If you are currently at or above 0.1 mg/L, it would be best to use Ultra Phos 0,04 to reduce PO4 even further. ZeoLight will help maintain a low PO4, but should not be seen as a dedicated method for reducing PO4. Once you've reached the desired range, the products that make up the system will help maintain those levels. If more nutrients are going into the tank than what is being removed, you will see an increase in phosphates.
Phosphates at 0.5 mg/L is high enough to keep the hair algae alive and growing. I recommend adding either Phos0,04 or Powerphos to your system as these will help lower your phosphates and control hair algae growth. For the Zeo-Light System, we recommend maintaining PO4 between 0.01 - 0.05 mg/L.
Cool, I've just purchased some Phos 0,04. I have also found some Ultra Bio in the back of the cupboard that came as part of the Zeo-Light starter set Just adding that too, as it appears to have an impact on PO4 too?
I have put 140ml of Phosphate 0,04 and my phosphate has dropped, but is now locked in the GHA, do i just continue to change every 3 weeks and it will be taken down to 0,04 overtime, or do i increase the amount in the short term to assist bringing it down?
Testing using a Nyos High Range test kit and its showing 0 PO4, which of course it isn't due to the algae farm on the rock? Any advise you can offer please?
There is likely PO4 trapped inside the live rocks and substrate. These are steadily released overtime. Maintaining your current PO4 media swap every 3 weeks would be best.
Do you occasionally vacuum the sand? A combination of vacuuming and partial sand removal can help export these PO4 deposits. We recommend removing some sand with each water change and replacing it with fresh sand.