I'm surprised that nobody has said anything about adding salt to the tank.
Never add salt directly into the aquarium with fish when trying to create a correct salinity. This causes massive salinity fluctuations and is like the equivalent of suddenly dipping the fish in salt water from freshwater which can be very risky. The actual undissolved salt too can burn the fishes skin and eyes.
If you do need salt, proper brackish aquarium setups require a hydrometer and an extra tank or bucket to premix the water. Always premix the brackish water. When raising salinity, do it over the course of weeks. I rose mine by no more than .001 in a day and .002 in a week. If I raised it any quicker, my fish stopped eating. While the fish do face changing salinities in the wild, it is not like being in a stream one minute, and then the ocean the next. Fish spend a lot of time swimming upstream to the ocean, or brackish environments. Also, if they are not yet ready for a sudden change, they just swim to waters they prefer. In a tank, this cannot happen.
I say keep yours in freshwater until it seems to need brackish water.