Fractional Chern insulators (FCIs) realize fractional quantum Hall topology in lattice bands, but their excitation spectra remain far less understood than their ground states. Here we establish a theoretical principle relating the periodic electron-density modulations of flat Chern bands to the many-body gap and excitation spectrum of FCIs. Contrary to the conventional view that such density modulations are detrimental to fractional topology, we show that different reciprocal-lattice Fourier components play sharply distinct roles: components at smaller reciprocal lattice vectors suppress the FCI gap, whereas components at larger reciprocal lattice vectors enhance it. By suppressing the harmful small-wave-vector components and amplifying the beneficial large-wavevector components, the gap enhancement can, in principle, be made arbitrarily large within the projected flat-band theory. Moreover, the same enhancement factor rescales the full low-energy spectrum, making the FCI excitation spectrum predictable from the corresponding Landau-level problem. We further generalize this correspondence to non-Abelian states. Applying this principle to moiré Chern bands, we identify these reciprocal-lattice density components as practical diagnostics for robust FCIs.