I have a one-gallon tank. The lid has a hole for feeding, etc., so water evaporates.
I have been topping it off with RO water per Petshrimp.com:
Today, I was having a conversation with someone on another opae ula forum that lead me to believe that the salinity will become diluted over time if I just keep adding RO water.Add RO or distilled water whenever the level of your tank drops by more than about 10%. You do not need to add any more salt because the salt stays in the tank, and the evaporation causes the remaining water to be saltier. How often you need to replace water will depend on the size and shape of your tank.
I really don't want to mess with changing the salinity. Per Mustafa's instructions, I have never checked my salinity since I mixed the water with the prescribed amount of salt at the onset.
I would love to hear from more experienced opae ula keepers than myself.
I set this tank up in March or April of last year. The shrimp seem to be doing fine, though I have never had a berried female. (I don't know if that's a measure of tank health, but I thought I'd throw that in there.)