Good news Steve! I had bad phosphate issues too, and I solved it by dosing Nitrates, as my Nitrates were 0, in the form of KNO3 (Spectracide Stump Remover). If you are interested in that, I can tell you how to mix it and how much to dose. Another thought as to why your chaeto isn't growing, could be related to Iron, I know that chaeto uses Phosphate, Nitrate and Iron to grow.