Sump for a 125 gallon.

[dino]

no set rule but 50 percent your main tank is always nice. also keep in mind your baffle height will determine how much water is in the sump also

[Alberta-newb]

As said before, the bigger the better! At a minimum, you need enough volume that in the event your return pump shuts off (power failure, pump fail..etc.) that the sump doesn't overflow.