Triploid fish sometimes grow faster and larger than diploids, making them valuable in aquaculture. Their underdeveloped gonads redirect energy from reproduction to muscle growth. This study compared embryonic development and antioxidant responses between allotriploid and hybrid diploid grouper (Epinephelus fuscoguttatus ♀ × E. lanceolatus ♂) after cold shock, aiming to support large-scale allotriploid production. The diploid group had a 99.08% fertilization rate and 82.87% hatching rate, while the triploid group had lower rates (95.71% fertilization, 39.63% hatching, 11.52% deformity). Triploids developed later, with higher incidences of abnormalities and mortality, especially in the gastrula and neuro-embryo stages. Triploid larvae exhibited shortened tails, abnormal yolk sacs, and impaired swimming. Flow cytometry showed triploids that had 1.55 times more DNA and larger red blood cells. Triploids had lower midgut trypsin and lipase levels than diploids (p < 0.05) but similar glucose, liver malondialdehyde, and total cholesterol levels (p > 0.05). However, they had higher liver and spleen lactate dehydrogenase, catalase, and alkaline phosphatase, along with lower spleen malondialdehyde and liver superoxide dismutase (p < 0.05). These findings offer insights into reducing mortality in allotriploid grouper aquaculture, aiding large-scale production efforts.