The valve is the air inlet adjustment. I don't really know how much of a system this skimmer could handle. Currently it's on a 250 gallon grow out system with supposedly a very high bioload. Skimmer manufacturers' "ratings" are basically wild guesses, usually based on how much money people with systems of a given size are willing to spend. And, tank size is hardly an accurate representation of bioload. The most rediculous of these I've seen is the berlin's claim to be appropriate for a 300 gallon system.