Effects of intermittent exposure to therapeutic levels of formalin on growth characteristics and body condition of juvenile rainbow trout

The effects of twice weekly exposure to formalin [formaldehyde] (200 mg/litre in a 1-h static bath) on juvenile rainbow trout Oncorhynchus mykiss (57.4 g initial weight) were assessed in a 12-week growth trial. Growth rates, appetite, feed conversion and body condition index of fish were not significantly affected by formalin treatment after 6 and 12 weeks. There was no evidence of a cumulative effect of formalin treatments over time, because the similarities between treated and untreated groups of fish persisted over the 12-week trial. Formalin-treated fish had significantly better fin condition and a lower incidence of corneal opacity than did untreated fish. It is concluded that formalin in a 1-h static bath at 200 mg/litre repeated twice weekly, can be used as a disease prophylaxis regime with no negative effects on growth..