02-14-2013, 12:33 PM
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As someone mentioned, the lone Serpae Tetra is hiding because it is stressed out by being alone. All tetra are shoaling fish, they live in large groups of hundreds, and they "expect" to have several of their species around them for security. They also have social interactions within the group. This species is best with no less than 8 fish, more being even better.
But here we come to some problems.
This is only a 6 gallon tank. There is insufficient space for this many fish, both from the standpoint of water quality and physical space. When this species, even more than most tetra, is in too small a space, it can increase the aggressive nature. The fish will literally nip each other to death.
I would strongly suggest that you consider different fish for this (small) tank, and hopefully return the Serpae. You can read more in our profile, click the shaded name. And browse the profiles for some ideas on suitable fish; minimum tank sizes and minimum numbers for each species are given.