Otos are difficult, if not impossible, to acclimate to a new tank. I had two out of nine survive. But once acclimated to your water source, then they can be transferred back and forth if you have multiple FW tanks. I put in extra StressCoat because they are sensitive to chlorine. Maybe you should age the water for a few days or even keep an empty tank of water with a filter running to use as a water source.