unknown; identification; help

BACKGROUND I have a 20 gallon long with neocardina shrimp, two hillstream loaches, and one nerite snail (which disappeared). I left home for about 40 days, placed an automatic feeder on the tank and had my neighbor check on them. I came back to a crazy amount of black algae on all my plants...

We recently started a tank and went to our local pet store to buy 6 scissortail rasboras.... however when we got home we had 5 scissortail rasbora and this guy... the only other fish in the tank were hatchetfish and he looks nothing like them. Any ideas? I don't want to take the poor guy back...