I was reading a thread earlier regarding skimmers and one of the posts mentioned sizing. So it peaked my curiosity; can a skimmer really be too large for the tank? If so, what happens? Physically, it could obviously be too large but if it fits, has an appropriate water level, and can otherwise function properly for the body of water it's intended to skim, will it perform just like any other skimmer?
Based on other things I've read, they can only pull approx 30% of DOC regardless; does the larger skimmer remove it faster? If so, what happens when it reaches that point (i.e. does it stop skimming for a period)?
If a skimmer is truly too large for a tank, we are not talking water volume but bio load. The skimmers intended "collection rate" will never be achieved and the size of the cone or cylinder near or in the collection area will never truly form a proper foam head and push enough skimmate out of the proper consistency. I have seen 30G tanks running on 300G rated skimmers and it works because the bioload is sufficient to keep the skimmer in the proper operating range.
Manufacturers go with this "Volume" rating because it is very familiar, intuitive, and makes the "upsell" easier. Also generally speaking larger tanks can and do support much larger bio loads so this is an ok rating system.
Generally speaking, a properly running and sized skimmer should always be producing something every day, of good consistency and color. but not to the point where you have to empty the collection area every day. If you can go a week without going passed the halfway point in your cup, you are in the ball park for your tank. If you aren't pulling any skimmate at all each day but there is foaming, then you are either oversized or something is not set right in your system; and then there is the chance that you just don't have the bioload to make the skimmer work. If you cup is full every day, with proper skimmate then you are severely undersized given your bioload.
Another thing to consider when picking out a skimmer is your sump size and flow, if you have an in sump skimmer, it only has access to the water in the sump, some skimmers are starved for bio load becasue tank/sump flow rate is to slow. With that being said, the larger your sump and the faster your flow the more access to biomass it has.
TL
R Skimmer Rating = Volume = Potential Bioload, So think Bio Load not Volume
I really don't know if it violates forum rules to link other forums but this is a great discussion I once read about the topic and really got me into skimmer sizing, sump sizing, skimmer compartment sizing, water dwell time, and biomass exchange with display and sump.
http://www.reefcentral.com/forums/showthread.php?t=2631250
