eels

Zigzag spiny eels stay pretty small (about 5") and should be ok. Make sure you don't have any fish that can eat it (my bichir ate mine :() Also they appreciate being able to bury in sand.
 
Just make sure that tank is covered. I have a small peacock eel (>4") in my ten-gallon now. If you don't have a hood, I would recommend an Eclipse hood, with built-in filtration.
 
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