Morin dye is known as a cheap and readily available selective ‘off → on’ fluorescent sensitiser when immobilised in a phase transfer membrane for the detection of Al³⁺ ions. Here, a morin derivative, NaMSA, which readily dissolves in water with good long- term stability is used in conjunction with a fibre optic transducer with lock-in detection to detect Al³⁺ in drinking water below the potability limit. The combination of a water soluble dye and the fibre optic transducer require neither membrane preparation nor a fluorescence spectrometer yet still display a high figure- of- merit. The known ability to recover morin- based Al³⁺ cation sensors selectively by exposure to fluoride (F⁻) anions is further developed enabling a complementary sensing of either fluoride anions, or aluminium cations, using the same dye with a sub- micromolar limit-of-detection for both ions. The sensor performance parameters compare favourably to prior reports on both aqueous aluminium and fluoride ion sensing.