Skimmer for a 40 gallon

[SAMSHUNG]

I can't suggest a good one however I can suggest one not to buy,
Red Sea Prizm <=== not the best skimmer.
I got mine for relatively little so it wasn't a big loss but I think I
should have paid a bit more and opted for a Remora skimmer instead.

You'll frequently find in this hobby that it pays to spend more.