I always suggest 10% water changes weekly (more frequently if having problems). So you should only need about 20 gallons (depending on the size of your sump) to do a water change.
I still don't see the problem. If you are getting 55 gallons in two days what is the issue? You can have your water sitting all week waiting to do a water change, you don't have to make it when it is time to do a water change. I would fill it, get the salinity and temp right and then (depending on the size of your pump) either keep a pump running in the trash can or run an air stone to keep the water fresh and mixed. This way you have water ready to go for a water change any time it may be needed.