Vinegar works but, i'm to impatient to soak for 1 or 2 days and you still have to use some elbow grease on stubburn areas.
Go buy a gallon of muriatic/hydrocloric acid from a swimming pool co. Lowe's. HD or hardware store. I maintain a large jug diluted with water to 20%. I clean everything from tanks, protein skimmers, pumps etc. with a quick 1/2 hr. soak a brief scrub and a good rinse with tap water. Don't be shy! it's the same acid you and the fish have in their stomachs from what I've been told.