Traditional farmers of milkfish and shrimp use 80% of Indonesia's shrimp production area, but produce only 10% of its shrimp. A coastal protection project funded a 16-day aquaculture field school (AFS) in order to train 277 farmers in Low External Input Sustainable Aquaculture (LEISA). Its cost was 1,060 USD per farmer. In 2017 and 2018, the project monitoring database completed records of yields and practices of 125 participants and monitored finances and water quality in a 10% sample. LEISA was adopted by 85% of the 125 participants. The two annual datasets of the sample were merged and trimmed from three outliers; this was done before statistical analysis. Compared to the baseline, LEISA adopters among the 125 tripled their shrimp' yields. Within the sample, the milkfish yields of adopters and non-adopters were about identical. Sampled adopters significantly increased their gross margin by 927 USD ha−1 year−1 due to three- and fivefold yield increases for milkfish and shrimp compared to that of the baseline. The rate of return was 1.3 for the 277 participants and 1.8 for the sample, indicating a payback time of <1 year. Enriching the farmers' skills with AFS can double Indonesian milkfish production and increase its shrimp production by 25–50%.