According to Slief neck size is the determining factor for size rating -- bigger is more.
In a matter of speaking, diameter/volume of the neck has a big impact on load size capabilities from one skimmer to the next and how that skimmer will perform in a properly sized situation vs a situation where the skimmer is oversized relative to the display load. I will also note that proper skimmer sizing should be based on display volume as opposed to system volume. This is because the loads are generated in our display where as our sumps and refugiums are there to reduce/export the load. Ultimately skimmer sizing is based on theoretical loads and it’s capabilities to export dissolved organics given it’s size.
The larger the neck volume, the greater the amount of dissolved organics that are needed to fill the neck with foam. It’s the dissolved organics (proteins) that allow the bubbles to bind into chains creating foam. The bigger the neck volume, the more foam needed to fill it. Typically, the larger the skimmer the larger the neck volume and often the larger the neck diameter.
In the absence of enough dissolved organics relative to the skimmer sizing, the skimmer won’t be able to produce enough dense foam to fill the neck. Instead it will generate faster moving bubbles that rise up the neck and burst at the surface and do very little to allow the proteins to attach to them in order to produce good skimmate. This is why skimmers that are grossly oversized to the displays loads typically perform inconsistently. They will skim great for a couple days until the level of dissolved organics are reduced. Then they idle along producing next to no skimmate or very clear skimmate until the organics build back up at which point they will skim again. The cycle will then repeat.
The only way around that is to run the skimmer really wet by raising the level inside the skimmer to the point that the line where bubbles turn to foam is far up into the neck. This not only makes it impossible to dry skim but it results in a skimmer that is very sensitive and prone to overflows.
As for overskimming.. In my experience, it’s kind of a fallacy. Testing from several years back has shown that even the best skimmers can only remove around 30% of the dissovled organics so it’s really not possible to over skim. That said, ultra low nutrient systems with lots of corals can benefit from additional organics that might other wise be stripped away from a good skimmer but this is all relative to the system and load. A system with a truly heavy fish load can benefit greatly from a well tuned properly sized skimmer where as a system with a very light fish load won’t always see the same benefits depending on the type of system.
It’s short, it’s been my experience that a properly sized skimmer will keep dissolved organics consistently lower than an oversized skimmer that is inconsistent and allows organics to rise and fall as the skimmer struggles to maintain a consistent foam head. As such, it’s important to take into consideration is the size range on skimmer ratings. At the low end of the size range for any given skimmer, it’s assumed that the load would be heavy. A heavy load by industry standards is typically about 1/2” of fish per display gallon. This holds true with most high end skimmers (some of which are conservatively rated) but obviously isn’t going to hold true to skimmers that are over rated and there are no documented guideline on which skimmers are over rated nor is their a correlation that I am aware of based on the pumps alone. One would have to compare body size and rating from a high end skimmer to the cheaper ones to see if the size of the skimmer matches the rating from one to the next.
As for comparing pump output, that is kind of difficult with skimmers that use variable speed DC style pumps since the pump ratings will always be based on the max capability of the pump. And often these same model pumps are used in a wide range of skimmer sizes from the same skimmer manufacturer. One of the advantage some of these variable speed pumps offer is the ability to fine tune the air/water flow going through the skimmer. Too much flow results in bubbles that rise too quickly and are too large while the excess water flow reduces contact time and can also result in unwanted turbulence inside the skimmer. Slowing the flow down improves contact time, improves stability of the foam head while also improving the density of the foam. So just because your skimmer came with a 50w pump that is rated at 2500 SCFH of air doesn’t mean it’s going to perform best with the pump running at full power. Even with fixed rate pumps, it really boils down to how much engineering and testing a company did when the skimmer was designed. I’m sure there are companies that copy well know skimmers and slap a giant pump on it which can be counter productive. It’s all about balance inside the skimmer. Too much water or too much air isn’t going to result in better skimmer performance.
At the end of the day, sometimes less is more.
