The production of high-quality seedlings is important to achieve better productivity in horticulture. Despite being well developed in conventional agriculture, there are few studies on aquaponic systems. The substrate choice in this integrated culture affects plant and economic feasibility. This study verified the effects of using Euterpe oleracea residue as a substrate for Coriandrum sativum seedling production in an aquaponic system with tambaqui, Colossoma macropomum. Five mixtures of Euterpe oleracea offal (0%, 25%, 50%, 75% and 100%) with coconut fiber, nourished by aquaponic wastewater, were evaluated. Water quality, fish and plant performance were analyzed during an 18-day experiment. Although total ammonia decreased from 5.17 mg L−1 to 0.64 mg L−1, nitrite and nitrate did not exceed 1 mg L−1 and 3 mg L−1, respectively. C. macropomum mean weight gain was 118.79 g and achieved high food assimilation, with a 0.86 feed conversion rate. Mean blood hematocrit (30.20 ± 5.99%) and glucose (59.5 ± 10.06 mg dL−1) indicates good physiological status of the C. macropomum. The use of the Euterpe oleracea offal mixture at 50% decreased by half the total fresh mass of Coriandrum sativum, down to 1.12 ± 0.30 g, but did not decrease dry mass (p > 0.05). When the mixture was above 25%, plant and leaf length were 8 cm and 3 cm smaller, respectively, and the number of leaves and stem base diameter were reduced by 34% and 40%, respectively. We do not recommend using more than 25% of Euterpe oleracea offal with coconut fiber for Coriandrum sativum seedling production in an aquaponic system.