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Algae control is normally a long battle. There are 2 articles on algae control in our articles section. Basically, control of nitrate/phosphate and cutting back feeding.
Algae is a long drawn out battle, even with assistance. I would keep running the phosphate media and manually remove what algae you can during your next water change.
I would recommend RO/DI water to ensure you aren't just putting back in nitrates/phosphates, since even RO water can have these in it. 10% weekly is a minimum when trying to get nuisance algae under control.
That depends on what the TDS reading of the water coming out of it is. That might be your problem...if the DI resin is used up and the filters need changed, all the stuff you are trying to remove is getting through.