Rhonda Murphy; Gregory A. Lewbart
North Carolina State University, College of Veterinary Medicine,
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.
We would like to thank Thomas Smith and Patrick Distefano for
assistance with this project and 5D Tropical Fish Farm for supplying the Corydoras
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,
3. Gratzek, JB. 1993. Parasites Associated with Freshwater
Tropical Fishes. In: MK Stoskopf, Fish Medicine, W.B. Saunders Co., Philadelphia, pp.
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