It may not be a "must have" but every tank I have and the tanks of my fellow reefers who's tanks look great have them. It's one of the most basic parts of nutrient export. You can absolutely keep a tank without a skimmer but in my experience that just means water changes have to be more frequent. The 4 most important components, in my opinion, are a heater, lights, skimmer and powerheads. Aside from the tank itself, these items are the main one's I that I buy the absolute best I can afford.
I've got a good friend that runs a 25g lagoon and his tank looks great without a skimmer. He's ran it for years and just ordered one yesterday because he got tired of doing water changes every week and still dealing with algae.
I've got a good friend that runs a 25g lagoon and his tank looks great without a skimmer. He's ran it for years and just ordered one yesterday because he got tired of doing water changes every week and still dealing with algae.