You might like to make the last chamber big enough for your heater - that way all the equipment can be kept out of the tank.
I am not sure why you need the first chamber, could you not have the water flowing straight in to a chamber with filter material?
I agree with Ira about the activated charcoal, its messy, doesn't do much and need changing frequently.
The only other thing I'd change is to put the filter wool first to act as mechanical filtration, otherwise your sponges and scouring pads will clog up quickly.