IMO, your tank is too new and too small for stars. Plus the sand sifting star will eat most of your biodiversity in your sandbed. I would use snails and gobies to sift your sandbed. 
 
Where the legs are missing, does it look like it was torn, or that it is dissintigrating? 
		
	
		
		
		
		
			
		
		
		
		
	
		
		
	
	
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