When you have a real phosphate "problem".. Or excessive phosphate also accompanied by a nuisance algae problem.....
Which could likely be never if you don't overfeed and don't have phosphates leaching from your rock (common with Pukani)..
Whats "excessive" you ask.. Well... Thats a tough question to ask and why I included the "accompanied by a nuisance algae problem"..
Many people have phosphate levels WELL in excess of commonly accepted levels around here without any issue at all..
Please watch this to see "whats up with phosphate" and pay attention to the levels his tanks are running at..
https://www.youtube.com/watch?v=ZRIKW-9d2xI