Help with changing from brackish to freshwater
 

So i'm cleaning out an uninhabited brackish tank with the intention of putting a frontosa cichlid in it. I have taken out as much water as i can but there's still about 1/2 an inch in there and i really don't want to remove and rinse the gravel, plants, tank, etc.

Is it absolutely necessary for me to do that, and will the salt be harmful to the new fish if i don't?

Thanks!

It depends on what salinity you were running the brackish tank at. If you were just using, say, 1 tablespoon per gallon of salt, then you will be fine just filling the tank from here. But if you were running at an actual brackish salinity, say specific gravity of 1.008 or higher, then I'd suggest reestablishing the biofilter with a complete cleaning.