what size tank is it,255ltr is too small for a big fish like the foxface(they are huge polluters)they ****e almost constantly,the low po4 reading is misleading as the algae is absorbing/using the po4 therefore the reading is what the algae cannot utilise therefore the algae will grow until it exhausts the po4 then sit at that level consuming the po4.reduce the bioload get the po4 and organics down the the algae will subside.hth mikk
p.s tang is not suitable for your tank they need more room mate.oopps re-read post you are not getting a tang.