A skimmer can be physically over sized and not make the oily, soapy looking foam to push it out into the cup, as mentioned it will just kinda fizz and deposit on the neck of the skimmer. It sorta still works, just not as intended.
It could also be over rated for the size of the aquarium meaning it will strip far too much out of the water under normal operations. It is possible to break the food cycle for your corals by extracting too much bacteria and planktonic life stages from the water column which may give you trouble down the road with a fully stocked tank.
I encountered this problem when I upgraded my skimmer, my LPS like chalice and micromussa suffered until I dialed it waaaaaay back from what I was used to.
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