Q. Is the fish tank itself Muktza on Shabbat?
A. Igros Moshe (O.H. 4: 16) considers the fish tank a ‘Bosis’ or a base for the fish it contains and therefore should be regarded as Muktza as the fish are. However, Shemiras Shabbos K. (27: n. 96) quotes Horav S.Z. Auerbach zt’l that a small tank is regarded an ornament and therefore not Muktza.
Horav Shlomo Miller’s opinion is that one should be stringent when possible.
See question 2810 regarding the extraction of a dead fish from an aquarium on Shabbos.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller, Horav Dovid Pam and Horav Aharon Miller Shlit’a