Your unknown fish is a mandarin goby that looks pretty skinny, they need live copepods found in an established tank.  
Some people have had luck with other foods, try a search on here. 
Normal salinity should be around 1.025. 
Are you running a skimmer? 
		
	
		
		
		
		
			
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				Brian 
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 220g inwall 48"x36"x30"
110g mangrove refug/sump
Poison Dart Frog Vivarium
			 
		
		
		
		
	
		
		
	
	
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