The 17th-floor penthouse in Gwathmey Siegel's "Sculpture for Living" building, a.k.a. the undulating glass offender of Astor Place, is on the market for $16 million. The apartment is special for a few reasons, not the least of which is its astounding use of fish tanks (1,300 gallons of 'em) as walls. The rather modern home is also not free from the hangups of other expensive New York City apartments, evidenced by its wood-walled study. With a fish tank as a headboard, at least who ever lives or will live here is never really sleeping alone.