Large Sump. What to put in it?

[whatcaneyedo]

What does your system lack? Theres no point in attempting to grow a macro algae if you're already using GFO effectively. Just as there wouldn't be much point in creating a deep sand bed down there if you've got an oversized skimmer and a boat load of live rock. Perhaps you don't have a frag tank... you could put frags down there. Or maybe you've only got one display tank. If thats the case then that extra space might be nice for a problematic fish or invert later on down the road. I like to keep a few extra pieces of live rock in my system for future use when aquascaping. Extra sump space is perfect for those. Cryptic refugiums are another option... the list goes on.