Received: Jul 17, 2025 / Accepted: Jan 08, 2026 / Published: Mar 31, 2026
Recirculating aquaculture systems (RAS) for rainbow trout (Oncorhynchus mykiss) have been widely implemented worldwide. However, their adoption in Vietnam has occurred only over the past decade and remains limited due to several operational challenges. Among these, disease management is particularly difficult because chemical treatments can disrupt the microbial community within the biofilter. This study assessed the effectiveness of a low-dose formalin treatment for controlling Gyrodactylus sp. infection in rainbow trout reared in a recirculating aquaculture system (RAS). During a mortality episode associated with Gyrodactylus sp. infestation, formalin was applied at concentrations of 15-20ppm for 10h per day over three consecutive days. Water-quality parameters, parasite infection intensity, and fish mortality were systematically monitored throughout each treatment period. The results indicated that after three days of treatment, fish mortality significantly decreased, while parasite infection dropped 95.3-96.7%. The results also revealed fluctuations in water quality parameters, including temporary increases in COD, N-NH₄⁺, and N-NO₂⁻ concentrations, associated with a decline in nitrifying microbial activity. Nevertheless, these changes remained within acceptable limits and did not cause severe system disruption. The findings demonstrate that low-dose formalin treatments effectively reduce mortality in infected rainbow trout without severely impairing the beneficial microbial community in RAS.
