Research and see what is generally recommended. This fish should be housed one per tank unless you have a mated pair. If those two were truly mated, it's going to be tuff to replace the lost one. You might want to just give this one a chance to adjust to being a loner, or give him up and buy a mated pair.
I don't know anything about http://www.aquacon.com/blennies_saltwaterfish.html">Aquatic Connection</a> and I'm not endorsing this supplier (I just did a quick google and it was the first hit), but they <u>do</u> advertise mated pairs. Other places might as well or you could get a LFS to keep a lookout for a pair.
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<span style="font-family: Arial;"><span style="font-size: 1-2px;"><span style="color: #ff0000;">Description: </span></span></span><span style="font-family: Arial;"><span style="font-size: 1-2px;"><span style="color: #000000;">The Golden Head Sleeper Goby, is known by the Pennant Glider, Yellowheaded or Golden Head Sleeper, or Blueband, Bluestreak, or Yellowheaded Sleeper Goby. Its head is yellow-gold with a sapphire-blue stripe below the eyes, and the rest of the body is off-white. </span></span></span>
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<span style="font-family: Arial;"><span style="font-size: 1-2px;"><span style="color: #ff0000;">Recommended Tank :</span></span></span><span style="font-family: Arial;"><span style="font-size: 1-2px;"><span style="color: #000000;">To be successfully cared for, it should be housed in a 50 gallon or larger aquarium with live sand as the substrate, and an attached populated refugium. </span></span></span>
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<span style="font-family: Arial;"><span style="font-size: 1-2px;"><span style="color: #ff0000;">Feeding and Diet:</span></span></span><span style="font-family: Arial;"><span style="font-size: 1-2px;"><span style="color: #000000;">It feeds off the bottom as it sifts through the sand eating mouthfuls of substrate and the food that lives within the sand, keeping the substrate well oxygenated. The Sleeper Gold Head Goby feeds on a variety of live and frozen brine shrimp, mysid shrimp, live black worms, and prepared foods for carnivores, and should be fed frequently. </span></span></span>
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<span style="font-family: Arial;"><span style="font-size: 1-2px;"><span style="color: #ff0000;">General Comments:</span></span></span><span style="font-family: Arial;"><span style="font-size: 1-2px;"><span style="color: #000000;"> May sometimes attempt to jump out., so we suggest a tank with a hood. It rarely will become aggressive towards other fish, but is territorial, and will fight with its own kind unless they are a mated pair. </span></span></span>
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<span style="font-family: Arial;"><span style="font-size: 1-2px;"><span style="color: #ff0000;">Level of Care: </span></span></span><span style="font-family: Arial;"><span style="font-size: 1-2px;">Moderate</span></span>
<span style="font-family: Arial;"><span style="font-size: 1-2px;"><span style="color: #ff0000;">Reef Compatibility</span></span></span><span style="font-family: Arial;"><span style="font-size: 1-2px;"><span style="color: #000000;">:Good Reef Compatability</span></span></span>
<span style="font-family: Arial;"><span style="font-size: 1-2px;"><span style="color: #ff0000;">Approximate Purchase Size:</span></span></span><span style="font-family: Arial;"><span style="font-size: 1-2px;"><span style="color: #000000;"> 2" to 3"</span></span></span>
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<span style="font-size: 1+1px;"><span style="color: #ff0000;">$29.99</span></span>
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<span style="font-size: 1-1px;"><span style="color: #000000;">A few</span></span><span style="font-size: 1-1px;"><span style="color: #000000;">Mated Pairs available</span></span>
<span style="font-size: 1+1px;"><span style="color: #ff0000;">$74.99</span></span>