Several veterinary medical reference texts and fish hobbyist's magazine articles state that tropical ornamental catfish belonging to the genus Corydoras cannot tolerate low salt concentrations.^2,3 To date we are not aware of a controlled study that supports this belief. Many freshwater ornamental fish appear to become stronger and healthier as a result of the addition of salt to their water.^1,2,3,4 The goal of this study was to determine if a popular aquaculture species of Corydoras catfish could withstand low salt concentrations. As a result of this study, aquarium fish hobbyists and the aquarium fish commercial industry will be able to question whether Corydoras catfish can tolerate low levels of salt in their water.

This 13 week experiment used a total of 80 Corydoras aeneus catfish and ten l0 liter aquariums, each functioning as their own closed system. Each tank contained 8°C. aeneus catfish, artificial sea salt, and 6 liters of de-chlorinated tap water. There were 2 tanks of each of the following salt concentrations: 0.0 parts per thousand (ppt), l.0 ppt, 1.5 ppt, and 2.0 ppt. The fish were fed a high protein diet of freeze-dried bloodworms every 48 hours.

The tanks contained no filtration during the first 2 weeks of the study. The husbandry protocol included a 67% water change and the monitoring of the pH and ammonia levels every 48 hours. There were 9 recorded trials of pH and ammonia testing during this initial period. The ammonia levels ranged from 0.5 to 1.2 parts per million (ppm) and increasing ammonia and salt concentrations could be directly correlated. Preliminary work with aquaculture finfish suggests a direct relationship between simulated shipping stress and increased water ammonia levels.⁵

The labor involved in water changes became prohibitive and sponge filters were added to each aquarium. Once the sponge filters became established (approximately 3 weeks), water testing was reduced to every 72 hours and water changes to once per week.

During the study, there was a 10 day period of inadequate heating of the catfish facility. Water temperatures dropped below the optimal 26 degrees Centigrade (the lowest recorded temperature was 14 degrees C) and 4 catfish died in different aquariums during this time period. During the 13 week study a total of 10 catfish died (3 from the 2.0 ppt tanks, 2 from the 1.5 ppt tanks, 3 from the 0.5 ppt tanks, and 2 from the 0.0 ppt tanks). There were no recorded losses among the catfish in the 1.0 ppt aquariums. Histopathology results are pending.

There is evidence that salt in the water may be stressing the C. aeneus but that this stress does not cause significant mortality over a 13 week period. Furthermore, 13 of 16 fish survived a salt concentration of 2.0 ppt for the entire study period and 16 of 16 fish survived in the 1.0 ppt water. Salt concentrations in the 1.0 ppt to 2.0 ppt range are considered therapeutic for ornamental fishes.^2,3,4 Future work will focus on evaluating the effect of salt on other species of Corydoras catfish including those not currently raised on tropical fish farms.

Acknowledgements

We would like to thank Thomas Smith and Patrick Distefano for assistance with this project and 5D Tropical Fish Farm for supplying the Corydoras aeneus.

References

1.  Gratzek, JB. 1992. Aquariology: The Science of Fish Health Management, Tetra Press, Morris Plains, NJ.

2.  Johnson, EL. 1992. Pass the salt. Tropical Fish Hobbyist, XL(12): 132-137.

3.  Gratzek, JB. 1993. Parasites Associated with Freshwater Tropical Fishes. In: MK Stoskopf, Fish Medicine, W.B. Saunders Co., Philadelphia, pp. 573-590.

4.  Lewbart, GA. 1989. High morbidity and mortality among wholesale level freshwater ornamental fish due to the ciliate protozoan, Chilodonella ~2. IAAAM Proceedings, 20:48-52.

5.  Tom Schwedler, Clemson University, Personal Communication.