Several things you can do under a main tank, or even next to it.
You can have a plain ole sump, you can have a wet/dry, you can have a refugium, you can do severel combinations if you like. A wet/dry basically has a bio ball chamber where arobic bacteria are housed on the bio balls, from there it enters a sump area of the wet/dry. Wet is the sump area, dry is the bio ball area.
I have a main tank that drains to both a sump and a refugium, the sump used to be a wet/dry, so I've used all three in combination before. I just converted the wet/dry into a sump by taking out the bio balls.
Thomas