"If I buy the corosex/calcite combination model N125, which media will be used more (which will dissolve faster)?"
Corosex generally dissolves at about twice the rate of the calcite. The main purpose of the calcite in the model N125 is to prevent the corosex from clumping together. Your #125 comes with more
calcite than corosex. Since corosex dissolves faster, when you reorder/install replacement media we suggest ordering/installing two parts corosex and
one part calcite.