It’s better to top off your aquarium gradually, as opposed to all at once. Over the course of a few days, your aquarium will probably evaporate a few gallons of water. Depending on the total volume of your tank, this can be a large amount. If you then dump a three or four gallons of freshwater to top off the evaporated water, it could cause a drop in salinity. Although it probably won’t be a huge drop, it could still cause some animals to show signs of stress. Instead of dumping all at once, try putting in only a little at a time. Spread it out over the day or multiple days to be safe.