Depends. Firstnthere are usually still trace amount of oils left on the skimmer from the press mold. once the oil wears off it will become more efficient. If it's a new tank you should probably have a relatively light load, so you won't pick much up from them either. However within 1 - 2 weeks you will start getting build up from left over fish food and fish waste, and you will start seeing junk in your cup.