Freshwater into saltwater for fish?

[Almostmikko]

can u add a little bit of freshwater to saltwater? say the addition is only .5 to 1 gallon? because my tank has a little gap between the top to the water and it kinda bugs me to see that.
i did a water change so idk what to do =[

 

[Pika]

Yes, if it's just from evaporation. Water evaporates from saltwater fish tanks and the salt stays.