Two-field fluid models for low-frequency density and electrostatic potential fluctuations in a purely toroidal magnetized low-β plasma are discussed. In particular, we present some simple models suitable for investigating plasma transport close to marginal stability. For gradient-driven fluctuations, linear normal mode analysis of the interchange and resistive drift wave instabilities are reviewed, with attention to the relative phase and amplitude of density and electrostatic potential fluctuations. This reveals many characteristic features of drift-wave fluctuations that are also observed in laboratory experiments and during ionospheric irregularities. Finally, these results are used to investigate the nonlinearly conserved energy functionals, discussing the effect of magnetic field curvature and particle collisions on energy, enstrophy, and cross-correlation between density and vorticity fluctuations. Implications for turbulent fluctuations and nonlinear transport in laboratory and ionospheric plasmas are discussed.