Abstract

Acute morbidity and mortalities were seen in several invertebrate and fish species over a 16-day period in multiple re-circulating saltwater systems at the National Aquarium. The first animals affected were eight seahorses (Hippocampus erectus) in an 85-gallon exhibit; clinical signs consisted of inappetance, increased respiratory rate and effort, increased activity, and mortalities (38% of group). Over the following few days, the animals in two adjacent exhibits showed clinical signs: gobies and hawkfish (Gobiodon and Neocirrhites spp.) in a 20-gallon warm, saltwater exhibit, and anthozoans, echinoderms, molluscs, an octopus (Enteroctopus dofleini), and surf perch (Cymatogaster spp.) in a 1400-gallon cold, saltwater exhibit. Life-support equipment was all running normally, and basic water quality (dissolved oxygen, salinity, pH, total carbonate alkalinity, total ammonia, nitrite) was within normal limits. At necropsy, no gross lesions were visible and no pathogens were identified. On histopathology, the predominant finding was branchitis. Initially, a local toxin was considered the most likely differential, and treatments consisted of supportive care, water changes, or moving animals to other systems. Multiple samples of the saltwater were submitted for full ion analysis, and other samples were frozen. By tracking the clinical improvement in some animals, and the progression of clinical signs in others, along with some water-source trials, it became clear that the primary issue was associated with the replacement saltwater. The full ion analysis of the raw saltwater was received 14 days after the first clinical signs. Potassium ion concentration was 13 mg/L compared to a target of 350-390 mg/L. Multiple water changes using a commercial saltwater mix, and following potassium supplementation of the saltwater batch, resolved the clinical signs over two days.

The cause of the potassium deficiency was a single batch of artificial saltwater in which potassium chloride had been inadvertently omitted. Clinical signs were first observed 12 days following introduction of the batch, and consisted of inappetance, increased respiratory rate and effort, activity changes (lethargy or excitation), and acute mortalities. While severity of clinical signs should depend on the rate of potassium decrease, and the species involved, an analysis of all the saltwater systems showed a consistent dose-dependent effect. Mild clinical signs were seen with potassium concentrations of less than 250 mg/L, and severe signs were seen in all animals at less than 150 mg/L. These lower levels were more common in smaller systems with a history of increased water changes. Unexpected issues with the source water should be considered whenever multiple systems and species are affected.