How Much To Reserve

i'm doing mu sump/ fuge and was wondering how much extra space should i leave incase of a power outage o the tank is a 55 gallon with 10 gallon fuge and 10 gallon sump

 

You should normally leave enough room to handle 2" of back flow from the DT which is 5.5 gallons. I would drill 2 anti-siphon holes about the same level as your overflow weir. I would also look into getting a 20 gallon as my sump if I were you.

 

you need as much space as the drawdown in your tank. I know that sounds vague, but fill the sump to the level that the pump does not suck air when on. then pull the plug and let it all equalize. one that is done, you can add water to it till it gets to whatever level makes your comfortable. for me, after all are off I am happy with 1" below the rim. then plug the pump in and let it run. once the level in the sump stops drawing, draw a line on the glass telling you your max level.

 

That is determined by the depth of your overflow box and returns.

I would say generally it would be 1 to 2" of the volume of the display.
If I remember right a standard 55G is 48" long and 13" front to back so 48x13x1"=624, 624/231=2.7 gallons per vertical inch in the display.
If those dimensions are right you need to leave somewhere between 2.7 and 5.4 gallons free for backflow.
It will really depend on your system though, the overflow teeth and returns in my 100G are 3/4" below the water level so I need to keep about 3.5 gallons of freeboard. 60"x18"x.75"/231=3.5"