I have a 120 gal tank with a 20 (I think) gal sump. Since getting it set up, it seems like I have enough room for a bigger sump and maybe a fuge. How big should the sump be for the tank? Is there some formula or a minimum?
Not so much a formula as much as a risk guideline. If the sump is too small:
It may overflow.
It may not stop microbubbles in time.
It may not hold your equipment (skimmer / heaters)
Larger sumps are great. They increase the water volume of the system, and hide everything. I always recommend using up all the space you can, and did exactly that when I made the sump that had to fit under my 55g. You can see it on my site, clickable from the front page.
Then I made a huge one earlier this year to put under my 280g reef that always draws a gasp in surprise when people see it.
