What a wild creature to find in your tank.
Very cool indeed !
Just a guess - it might be an American Eel (Anguilla rostrata)
copied from webpage:
The eel begins and ends its life in the waters of the Sargasso Sea, an area north of the Bahamas.
The leptocephalus, a pelagic larvae of less than two inches in length, drifts with the ocean currents for 9 to12 months before entering coastal waters.
When it reaches approximately 2.4 inches in length, the leptocephalus metamorphoses into a transparent, "glass" eel.
In autumn the glass eels migrate into estuaries along the Atlantic coast, including Chesapeake Bay, where they become pigmented. These eels are known as elvers
<a href="http://www.chesapeakebay.net/info/american_eel.cfm" target="_blank">http://www.chesapeakebay.net/info/american_eel.cfm</a>
<a href="http://www.ecoscope.com/eelscope.htm" target="_blank">http://www.ecoscope.com/eelscope.htm</a>
<a href="http://www.ecoscope.com/eelbase.htm#american" target="_blank">http://www.ecoscope.com/eelbase.htm#american</a>