Additional filter

Mind if I throw in some cents? I'm not sure what your filter media set up is, but if you have it, ditch the carbon if the tank is well established and just pack it full sponges, ceramic and live rock. Reducing the flow by adding a pre-filter will most likely decrease your filter efficiency. With both of my canisters it's only sponge and ceramic, with live rock in one of them and they do a fantastic job. lots of sponge means lots of area for bacteria that will grow and break down a good amount of the garbage it sucks up. Just for a little overkill in my 75 I have a reverse undergravel system which is attached to power heads with a pre-filter sponges. Yes I did just advise against them, but in that case they work well to break down things into little bits and then the bacteria under and in the gravel does the rest. Just my opinion however.

Live rock. Are you sure it's live rock? And if it's live rock, are you talking about using live rock in a freshwater setup?

Mine is a freshwater setup. I thought it was live rock. I came with the setup when I bought it from the guy. It is like white coarse stones. So no, not sure. It may just be crushed rocks haha, I guess I just assumed that's what it was.

I imagine they're just little stones he used as the first stage of filtration. They can do a pretty good job of catching the big stuff that comes into the filter and also serve as a rudimentary bed for bacteria. I know a guy that used rocks like these to weight down the underside of his sponge filters and provide some coarse filtration.

you can have live rock as well as deep sandbeds in FW aquaria..
the bacteria in saltwater is different than fw but the purpose of the species is pretty much the same.
in essence given time live rock in FW will become live rock and will work to complete the nitrogen cycle very similarly as in SW.

however, keep in mind that these species of bacteria live in an oxygen deprived environment...and for all intensive purposes would not live in the rock in a filter(prob not large enough to support O2 deprived environment and still fit in the filter and still have room for other media)..more than likely that rock will suffice as partially a mechanical filter and alternatively as biological media for the surface dwelling bacteria in the first 2 stages of the nitrogen cycle.