It depends on what's in your tap water. If it's chlorine, then yes, letting it sit will do the job. If it's chloramine, that's not going to evaporate, and you'll need to buy some Prime. It's not really that expensive -- a $40.00 bottle of Pond Prime will last you a good long while.
You definitely need to do water changes -- I do mine the same frequency and percentage as on my tanks -- 25% every two weeks, testing the week in between.