I used to have a 25 gallon tank. It was home to a pair of Black Ghost Fish (Apteronotus albifrons) and something that cleaned the glass. They would hide in daylight, coming out at night...where they'd dance. They'd swim in unison, creating the most amazing geometric patterns. I would watch them for hours. Their diet was brine shrimp and it wasn't uncommon for them to take it from my hand.