You should "make-up" for evaporated water every day or more often if possible. ADD ONLY WATER NOT SALT WATER MIX!!!! (The water evaporates - the salt does not). For your 55 gal, try to change 12-15 gal per month. You can do this all at once or if more convenient, change 3-4 gal each week. Make the mix water the night before. For my 120, I change 10-12 gal. every Sunday morning. I like doing changes weekly. This way I can clean the glass, all the filters, etc. I feel it helps keep all the water parameters more stable. Good Luck!