If the algae looks like little feathers, then it's actually
bryopsis, and is a real bugger to get rid of... I'm glad you've been successful at eradicating it.
brian_e tore down his 360g tank because he had a huge bryopsis problem and couldn't gain any headway... it was really bad... so consider yourself to be a lucky fella.
I agree with you that a sponge isn't the best idea, because it leaves a spot for organics to build up. I also don't recommend live rock rubble, because it's a huge detritus gathering spot too. I used to have a rubble pile in a spot in my old sump, and it was a detritus magnet... it slows the water down
just enough that the detritus settles out in that area, and it's a real bugger to clean, especially once the detritus settles 'into' the rock pores and starts causing problems with nutrient levels.
I'd put the heater in one compartment, the temp probe in another, and then your pump in the last one... but that's just me. Otherwise I'd leave it open, and try not to put anything in there that's going to trap detritus. Keep in mind that you do, however, 'want' detritus to settle in this area, so it stays out of the display portion of the tank, but you also want to be able to clean
all of it out when you do a water change... adding in rock/sponges, etc. just makes that job more difficult (if not impossible).
Good luck.
