Monitoring dissolved ammonia (DA) in tilapia fish farming ponds is important because a DA concentration as low as 100 parts per billion (ppb) depresses tilapia food intake and growth. Hence, a DA sensor capable of sub-ppm-level detection based on the Förster resonance energy transfer (FRET) of a single donor–acceptor pair has been developed to determine DA in tilapia breeding ponds. Certain mole ratio amounts of ruthenium organometallic complex (donor) to bromophenol blue (acceptor) were immobilized in hydrogel, polyvinyl chloride, and polysiloxanes membrane matrices. The hydrogel membrane matrix showed the best performance. A DA sensitivity at the ppb level has been achieved on a membrane of 2–3 µm thickness. The FRET sensing membrane performed reversibly with a response time of 10 min at 100 ppb DA in phosphate buffer (pH 11) with 100% (8 ppm) dissolved oxygen content in the solution. The pH and other amines did not interfere with the selectivity toward DA. Hence, the membrane has been applied to determine the DA concentration in tilapia fish ponds with 80% accuracy as compared with the Nessler method.