i use arm in my reactor and keep the ph at 6.8, arm dissolves at higher ph values my cal stays between 400 and 450 with 8 clams and a host of sps and lps corals, the arm dissolves nicely. that's the great thing with using arm. the bad thing is unless the cal reactor is a reverse flow(and geo's are ) the arm tends to dissolve into kind of a paste and may clog the pump and reactor up. this especially happens if the ph is run to low