I've had many, many moss balls...
But never tested for phosphate...
All of my fish, snails & shrimp love them though!!
Between my 6 tanks, I have about 9 or 10.
Plus have one (my cat tore up when I had it "vacationing" in a small cold water jar) I took the pieces and glued them onto cholla wood and it is doing great!!!
Bright green and the shrimp especially love it!!
Good luck!! I love my moss balls.
Was my first aquatic "plant" lol. They're EASY! and seem to thrive in all of my tanks and even under different conditions!!
Hope you enjoy yours!!