as much water as you can transfer from the 30g plus the bacteria in the filter on your 30g should be enough to allow your fish to safely settle into your new 55g. Transfer your fish into a holding bucket with an air stone (if your have one). Then transfer what water you can to the 55g and allow the filter to run for 1-2 hours (more if you feel necessary), unless you have any extremely sensitive fish they will adapt to the changes in no time!