Singapore is small enough that it’s just minutes to the sea from anywhere on the island
I tried using sea water a friend collected for me in the breeding tank for the lohena larvae. They still died within days. No different than if I made the water with marine salt myself. I notice the larvae are attracted to light and swim towards it. So my view is that they don’t swim to the darker parts of the achaline ponds but likely towards out to sea or if not, brighter areas of the pond. The adult lohena tend to stay at the bottom or hide in crevices . In comparison My opae ula swim all over the tank including the top.