That type of algae looks like the outbreak I had about a year and a half ago, when my sandbed started crashing. Here are some reasons for this algae (or any) outbreak.
- Your sand bed may be crashing (There are a lot of variables that can lead to this). It could be leeching phosphates or any other funk back into the water which basically negates your work at trying to get rid of it. What's the history on the sand?
- Your rocks may be leeching phosphates into your tank... what is their history?
- How old is your RO
system, also why are you using two DI
cartridges? Maybe your RO
membrane is too old and is lacking at filtering out silicates and what not from your tap water?
- What's the livestock in your tank? Maybe your fish capacity may be a little more than the tank or it's system can handle which causes a threshold of funk levels in your tank more quickly to reach an algae growing point?
On a personal note I don't very much trust PO4
test kits, and if I did I would trust from a higher brand like Salifert or something similar. Anyway I hope this helps out some.