Well, leave the fish in the tank and set up the 75 gallon. Turn on the filter and heater (optional) and let it run for 10+ days. Then, after 10 days, have 4-5 buckets and put the fish in. Make sure you put the right fish together because some fish can get aggresive with eachother in small areas. So make sure you have a compatible number of CLEAN buckets filled with fresh, clean water at the right temperature. Now that the 75G is cycled, and the fish are in the buckets, remove everything from the 55G that you want in the 75G, and put the stuff in the 75G. Anything you bought from the pet store such as live plants, put them in right now. Shut the 55G down. make sure the temperature in the 75G is compatible and carefully introduce the fish to their new, larger tank!