Under what conditions does algae come back?

[JasonLion]

Yes, turn off the SWG and do an overnight FC loss test.

 

[chem geek]

Do the phosphate tests only act on certain types, like fertilizer derived ones?

Phosphate tests only test for inorganic phosphate aka orthophosphate and that's also the only kind of phosphate that phosphate removers remove. So organic phosphates are not detected in the tests nor are they removed by phosphate removers. Algae can grow by utilizing organic phosphates though they are slower in uptake than inorganic phosphate. This is why I say that phosphate removers should be seen more like algaecides that can slow down, but not completely stop, algae growth.