Most phosphate test kits are utterly worthless. The only one I've gotten anything close to what the lab tells me is Hanna's. They're like $50 though.
AlgaeFix by AP is the same exact stuff as MRLs, though MRL is a slightly higher concentration (5.5% vs 4.5%). This stuff is absolutely lethal to shrimp, crays, etc. I'm surprised that you had an amano shrimp survive, Viktor, but they're pretty hardy buggers. I've had it kill CPOs, cherry shrimp vars, and amanos. (Admittedly, I've never used the MRL brand, but it is the same stuff).