The only concealed skimmers I know about are the ones on those expensive tanks with all the filtration fitted in the section on the inside, back of the tanks.

Like on the Red Sea Max Nanos

__________________
8' x 2' x 2' - Marine Reef.
6' x 2' x 18" - Wild Caught Zaire 'Moba' Frontosa
4' x 2' x 20" - Malawi
4' x 22" x 18" - Malawi + Victoria
4' x 18" x 18" - Breeding Cuban Cichlids.
Trigon 190 - Male Paratilapia Polleni
20" x 20" x 20" - Marine
30" x 1' x 1' - Hospital Tank
2' x 1' x 1' - Tropical Community

and

7x - 1' and 2' cichlid fry tanks