There's a calculator on a site called SaltyZoo that will work out the number, volume and salinity of water changes to make. you said 55 gallons, didn't mention a sump, making a guess for displacement of rock and stuff inside the tank, I guessed about 40 gallons of actual water.
I set the starting salinity at 1.028, and the target at 1.025. This will put you right in the middle of the "usual" 1.024-1.026 range.
Basically it calls for 4 changes of about 1.1 gallons each, of plain RODI.
This plan calls for 4 water changes
[Number] [Tank Sal. (BEFORE)] [Gallons] [Salinity] [Tank Sal. (AFTER)]
[1] [1.028 sg / 37.1 ppt] [1.1] [1 sg / 0 ppt] [1.027 sg / 36.1 ppt]
[2] [1.027 sg / 36.1 ppt] [1.1] [1 sg / 0 ppt] [1.026 sg / 35.1 ppt]
[3] [1.026 sg / 35.1 ppt] [1.1] [1 sg / 0 ppt] [1.026 sg / 34.1 ppt]
[4] [1.026 sg / 34.1 ppt] [1.1] [1 sg / 0 ppt] [1.025 sg / 33.2 ppt]