A diffuser for oxygenation of aqueous media comprises a manifold connectable to a source of pressurized oxygen-containing gas and having a plurality of outlet ports communicating a hollow interior of the manifold with an exterior thereof. A plurality of flexible microporous tubular membranes each have a coupled end secured to the manifold at a respective one of the outlet ports thereof to communicate an internal passage of the tubular membrane with the hollow interior of the manifold. An elongated rod extends along each tubular membrane within the internal passage thereof to maintain a length of the tubular membrane in an orientation projecting outward from the manifold with the free end of the tubular membrane further outward from the manifold than any other portion of the tubular membrane. The rods prevent curling of the tubular membranes over time when the diffuser is submerged in a liquid medium for use.