red algae?

[Origami2547]

0.5 on phosphates is high for a reef tank. Among other things, it will fuel algae growth and deter coral growth.

Almost certainly the problem you're facing is cyanobacteria (aka, red slime algae, cyano, and bluegreen algae). It's a primitive life form that is very versatile in what it can use as food. It's not all about phosphates, in other words.

Read this on cyano: BlueGrAlgae

Typical causes: 1) Lack of competition from more advanced life forms due to an immature system (this is why cyano is commonly part of "new tank syndrome"); 2) high levels of dissolved organic compounds, aka DOCs (which can be addressed by feeding less, skimming more heavily, using GAC - granular activated carbon - or more frequent water changes; 3) poor flow or stagnant areas in the tank; 4) poor lighting - or old lighting - where the spectrum does not support the development or sustenance of competing photosynthetic organisms, thereby making nutrients available to cyano. 5) high nitrates or high phosphates (often indicative of overfeeding)

[drakejo2002]

ok, i have an old canister filter, can i use this as a phosphate reactor?