Overskimming is a myth. Skimmers really can't remove much more than 30% of the dissolved organics in the water. The big issue with running a grossly oversized skimmer is consistency. The bigger the skimmer, the larger the neck. The larger the neck, the greater the amount of dissolved organics will be needed to fill the neck with foam to generate skimmate. If you don't have enough dissolved organics to fill the neck, the skimmer won't produce skimmate unless it's run REALLY wet which can result in overflows and very light colored skim. If the skimmer is inconsistent, what happens it that the skimmer will produce for a couple days and then idle along and not producing. During this time, dissolved organics will increase in the water instead of remaining consistently low. The end result is that the organics in the water will rise over the course of a week or more until there is enough dissolved organics in the water to generate a good foam head again. Generally speaking, this is less than ideal since excess dissolved organics can contribute to other issues such as nuisance algae. Now some skimmers are grossly over rated while others are under rated which kind of muddies the waters.. I've always said that you're much better off with a properly sized skimmer that is always working hard and is consistent as a result than an oversized skimmer that hardly has to work and is inconsistent as a result.
