Aqua Clear Pro 75 with set up

Hello all,
I just bought a 125 gallon tank. It came with an Aqua clear pro 75,
with skimmer, overflow, basically the whole nine yards. Does anybody
have a link on how to set this thing up? The original owner does not
have the manual. I do realize that it is not big enough for the 125 and
I will be using a second filter as well. The guy gave me thee power
heads with the tank as well. One is an Aqua Clear 901 and I have two
801s. There are no bioballs with the filter. There are two hard coral
looking 1 inch thick sponge looking bricks on the second level of the
filter. What are these? Originally it was set up as a salt water rig
and I am going fresh water. Please help....