I'm pretty sure mud skippers are brackish but I'll have to see... Also look at the tank size and other things.
Yes, mudskippers are brackish. I had them in the 1980's, fascinating little fishies. But they need brackish water, and space to swim and crawl/jump about. A terrarium-sort of habitat not a basic aquarium. They spend most of their time out of the water.
Opened the tank cover to feed them one evening, and the little rascal got so excited he jumped out and I was chasing him across the carpet! Didn't phase him in the least.