Skimmer sizing is a huge debate. Most skimmers are rated by how much water can be processed in a given amount of time similar to turnover rates for sumps. Most manufacturers use 3 times the system volume per hour for normal or medium bioload.
As the article referenced above shows, over skimming a tank is a myth. When the total TOC's in the water column fall below a certain amount the skimmer simply stops being able to produce a foam head that can reach the collection cup. When the TOC's rise again the foam head will build again and be able to reach the top of the cup and be removed.
In my opinion a skimmer is properly sized when it reliably skims but periodically stops. If it is constantly foaming it's not keeping up with the TOC production in tank and should be upsized.
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