I've housed numerous species of eels (Including the Gymnomuranea zebra you mentioned) in salinities as low as 18ppt long-term without incident. Have treated tanks that contained eels (Gymnothorax funebris, G. vicinus) at 15ppt for 30 days, for what it's worth
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Barrett L. Christie
Senior Aquarist- Quarantine
A Public Aquarium
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