Department of Microbiology and Immunology


Calculations of disease treatments

a) Water treatments are based on water volume. A predetermined amount of chemical is added to the water for a specified time. If an inadequate amount of chemical is added to the water the treatment will be ineffective. If too much chemical is added to the water or if the fish are left in contact with the chemical for an excessive period of time, the fish may suffer from chemical toxicity.

b) The amount of treatment chemical added to the feed or injected into the fish is based on the weight of the fish. Improper doses may result in an ineffective treatment or mortalities.

c) Aquaculturists should compute the volume of each culture unit (e.g., pond, tank, raceway) before a problem occurs, preferably when the system is designed or filled with water for the first time. The information should be stored so it is immediately available when needed. Practice calculations should be done so the culturist is comfortable and familiar with the computation procedure. A useful text, "Handbook for Common Calculations in Finfish Aquaculture" by Gary Jensen, is available from the Louisiana Cooperative Extension Service, Baton Rouge, LA. ($3.50). Culturists are strongly encouraged to obtain and use a copy of this or a similar work book. If you lose or gain a decimal point on a sample problem you get the answer wrong. If you lose or gain a decimal point in real life your treatment will likely be ineffective and your fish will continue to die or you may actually kill your fish with the treatment!

Sample Calculations

To provide the culturist with the opportunity to become familiar with the methodology used to calculate fish disease treatments, two hypothetical situations are presented. Calculations and steps are shown in detail.

Example 1.

You have a raceway with rainbow trout that are infected with the parasitic protozoan, Ichthyophthirius. You elect to treat with copper sulfate (CuSO4). The raceway contains 5000 gallons of water with an alkalinity of 75 milligrams per liter (mg/L or ppm). How much CuSO4 would you use?

Step 1.

Examine Table 1 and determine what concentration of CuSO4 should be added to the system to provide an appropriate treatment.

You know that the alkalinity is 75 mg/L. Therefore, the appropriate treatment concentration for CuSO4 is 0.5 mg/L.

Step 2.

Determine the quantity of CuSO4 to be added to the raceway to achieve the 0.5 mg/L concentration:

Treatment Desired X Conversion Factor X Water Volume X Correction for % Active Ingredient =  
0.5 mg/L 0.38 g/100 gal 5000 gal 100/100 9.5 g CuSO4
in the 5000 gallon raceway

Note: Because Ichthyophthirius has a life cycle that must be considered in the treatment of this organism, a treatment for this parasite usually consists of applying the treatment chemical every three days for 3 to 4 treatments.

1Conversion factor is the amount of material per 100 gallons of culture water needed to provide a 1 mg/L (or ppm) concentration.

[1 mg/L (or ppm) = 0.38 grams (g)/ 100 gallons (gal)]

2Percent active ingredient is the amount or purity of the chemical that is effective in treatment of the disease. Copper sulfate is usually available in a 100% active form (pure copper sulfate), but many chemicals are not. If the chemical is not 100% active, a correction factor must be generated:

100% / percent active ingredient = correction factor

The percent active ingredient information can be obtained from the label on the container in which the chemical was supplied or from personnel at the diagnostic laboratory.

Example 1 (continued)

If alkalinity in this same example had been below 50 mg/L, CuSO4 would not have been the treatment of choice due to potential toxicity to the fish (Table 1). An alternative treatment would have been formalin. The treatment concentration for formalin is 25 mg/L (or ppm).

Treatment Desired X Conversion Factor X Water Volume X Correction for % Active Ingredient =  
25 mg/L 0.38 g/100 gal 5000 gal 100/100 475 ml formalin
in the 5000 gallon tank


Note: Although formalin is 37% formaldehyde gas dissolved in water, for fish treatment purposes this material is considered to be 100% active.

Example 2.

You have carp in a garden pond with a diagnosed bacterial (Aeromonas hydrophila) infection. The pond contains 10 fish that weigh an average of 2 pounds each, for a total of 20 pounds of fish. The bacterium is sensitive to terramycin. How would you prepare the treatment?

Step 1. Calculate the amount of drug to use:

Terramycin is used at 2.5 grams (g) active ingredient per 100 pounds (lb) of fish per day for 10 days. The drug is mixed with the feed and fed to the fish. You have 20 pounds of fish, so you need:

Treatment Desired X Pounds of Fish X Length of Treatment X Correction for % Active Ingredient =  
2.5 g/100 lb fish 20 lb fish 10 days 100/50 10 grams of terramycin


Step 2. Calculate the amount of food that you will feed to the fish during the 10 day treatment period:

Ornamental fish might reasonably be fed at a rate of 1% of their body weight per day.

Pounds of Fish X Feeding Rate X Length of Treatment =  
20 lb fish 0.01 10 days 475 ml formalin
in the 5000 gallon tank


Step 3. Commercially prepared medicated feeds are available and can be used. Alternatively, medicated feed can be prepared as follows. Mix the 10 grams of the terramycin with 2 pounds of feed and provide 0.2 pounds of medicated feed to the carp each day for 10 days. In many instances the antibiotic is mixed into a vegetable or fish oil to make suspension of the antibiotic in the oil. The feed pellets are then covered with a thin film of the oil.

3Terramycin premix often is supplied as a 50% active mixture. Always check the label of a specific package.

4The feeding rate is the percent of body weight used to calculate the amount of food for a daily ration, converted to a decimal format:

0.01 = 1.0% daily feeding rate / 100