If a variety $V$ contains all  algebras derived by
hypersubstitutions, then it is called {\it solid}. If the only
derived algebras obtained from an algebra $\mathcal A$ in $V$ are
those isomorphic to $\mathcal A$, then the variety $V$ is called
{\it fluid}. We generalize this concept to the concepts of
$n$-fluid, weakly fluid and $\aleph_0$-fluid varieties, prove some
general properties and determine all $n$-fluid
varieties of bands.