maybe im unlucky. tap or seawater, both boom in my case. 3 weeks for tap water is somewhat risky. i wasn't the patient type, did not heed advise from the shop and cycle for approx 2-3 weeks only, my tank crashed. total live stocks gone in that crash cost me a bomb and within days only.
been using tap water for my tanks. algae boom is a norm in all new tanks no mater what water you use. but if you gonna use tap water plus salt for new start up, minimally cycle it for at least 1-1.5 months. seawater for a faster process.