The phosphates and most likely some hidden nitrates are the cause here and what you are seeing appears to be early onset to more Cyanobacteria, rather than algae. But with that said, both algae and cyano
come from the same thing, excess nutrients in the water column.
There are two articles in my signature that discuss these battles, but understand that the 50% water change only cut your levels in half. That .25 phosphate was .5 before the water change. So, more large water changes with ro
water is in order. This is why algae and cyano
battles can last for some time.
Sent from my iPhone using Aquarium Advice