Well honestly you can put what ever you want in your fuge. You can put live rock rubble, sand, macro algae, or just about anything else you would put in a saltwater tank. a fuge is simply a safe haven for something(s) that you cant grow in your main tank. For example a fuge full of rock rubble will be great at growing pods
and bristleworms. A fuge full of macroalgae will be great at removing excess nutrients in the water. A DSB
fuge will be good for helping the denitrafication process.
In your sump you can put your heaters, skimmer, uv
sterilizer (if you have one), ozone machine (if you want one), calcium reactor (once again ifyou have one).