Do you have a link to the model? Are you just putting rocks in it or macro too?
If you want to reduce the flow you can use a smaller impeller or break a blade off depending on the impeller. you can also put a sponge pre filter on the intake.
Put ceramic rings or rock rubble in a media bag or without one if you prefer and drop it in the filter. There different ways you can set it up it depends what you want to achieve from it.
I just set up a couple of aqueons. The ones I have contain two compartments. The first is for filter floss the second has ceramic rings or pot scrubbers. I made it into a cryptic zone. I was reading and article about them and decided to try it out.