No filter here, not since I sold the Rockwood in 2007. It had an inline filter inside a clear housing. After one summer of use that filter turned icky green with stuff growing in it. Nothing growing in the water tank or lines that I could see, just in that filter. If you do get a filter, get one with something that will prevent growth. KDF will do that, or impregnated silver.
Speaking of silver, I think a silver coin in the bottom of the water tank will kill bacteria. Settlers crossing the plains in the 1800s would put silver coins in their water barrels.
To every thing there is a season, and a time to every purpose under the heaven... --Ecclesiastes 3