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Formalin Flow-Through Treatments for External Parasites

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Trout commonly have some external parasites. But when the conditions are favorable, these parasites can flourish, causing substantial trout mortalities. Visual signs of parasites are easily recognized once you are familiar with the symptoms. “Jumping”, “flashing”, and “darting” (erratic bursts of activity) by the trout can be signs of external parasites.

The external parasites of trout that are a concern in North Carolina include Costia (Ichtyobodo), Ich (Ichthyophthirius multifilis), Episistylis, Trichodina, Ambiphrya (Scyphidia), and Gyrodactylus. All of these parasites can usually be controlled with one flow-through treatment of formalin (Table 1) except Ich. Ich has three life stages, one life stage is not susceptible to formalin treatments. For this reason, control of Ich requires three or four treatments with formalin at intervals based on the water temperature (Table 2).

Before applying a formalin treatment, it is best to use salt first. Due to the irritation caused by the parasites, trout increase mucus production. Under these conditions, the parasites can become surrounded by mucus, decreasing the parasite’s susceptibility to the formalin treatment. Prior to the formalin treatment, apply 1 pound of salt for every 2 gallons per minute of water flow. About an hour or more after applying the salt, begin the formalin flow-through treatment. Minimize the stress the trout are subjected to by treating when the water temperatures are coolest in the summer months and keeping the fish off feed for 1-2 days prior to and during the treatment.

Generally siphoning or drilling a hole in the bottom of the formalin container will not deliver the formalin at a uniform rate. A turkey fount (waterer) can be used to deliver the formalin at a uniform rate. To dispense about 7 gallons of liquid, drill a 3/32 inch hole. The hole may need to be enlarged slightly to dispense 7 gallons in one hour. Calibrate the formalin dispenser to ensure a known amount is delivered in one hour. Various size turkey founts can be found at most agricultural supply stores or catalogs. A 7-gallon turkey waterer costs about $50.

formalin dispenser

Table 1 – Formalin Flow-Through Treatments for One Hour

for External Parasites on Rainbow Trout

* * Below  50 degrees F * * 50 – 65 degrees F * 24 hr
* * 250 ppm * * 170 ppm * Treatment
Water Flow * (1/4000) * * (1/6000) * 25ppm
gpm gal oz ml gal oz ml gal

50 * 96 2839 * 65 1930 1.7
60 * 115 3407 * 78 2316 2.0
70 * 134 3974 * 91 2702 2.3
80 * 154 4542 * 104 3089 2.7
90 * 173 5110 * 118 3475 3.0
100 1.5 192 5678 1.0 130 3861 3.3
200 3.0 * * 2.0 * * 6.7
300 4.5 * * 3.1 * * 10.0
400 6.0 * * 4.1 * * 13.3
500 7.5 * * 5.1 * * 16.7
600 9.0 * * 6.1 * * 20.0
700 10.5 * * 7.1 * * 23.3
800 12.0 * * 8.2 * * 26.7
900 13.5 * * 9.2 * * 30.3
1000 15.0 * * 10.2 * * 33.3

* at warmer water temperatures, it may be necessary to treat for longer periods at low concentrations (i.e. 24 hours at 25-35ppm)- Do not treat formalin-sensitive trout

– 128 ozs = 1 gallon

– 3785 ml = 1 gallon

– For additional flows (i.e. 1250 gpm & 250 ppm), add treatment volumes:
15 galllons + 3.0 gallons + 96 fluid ounces = 18 gallons 96 fl. ozs. or 18.75 gallons

– For some parasites (Ich), repetitive treatments may be necessary. Use the following information as a guide for treatment intervals. You may need to consult with a fish health professional.

Table 2 – Formalin Flow-Through

Treatment Interval


Temp oF



65-74 Skip 1 Day
55-64 Skip 2 Days
45-54 Skip 3-4 Days

(Some of the information for this article was obtained from Andrew Mitchell, Fishery Biologist, U.S. Fish and Wildlife Service, Stuttgart, Arkansas)