Originally Posted by Sniperhank
Wet/dry does the exact same thing that happens inside the porous live rock when the tank is cycled. It is a place for bacteria to live to support the nitrate cycle.
EDIT: I forgot to mention that if not cleaned weekly, they become a nitrate trap.
What about a good size sump for a 75g. I read that it should be at least 25% of the display tank. Is that a good number to base it off of?