I'm looking to start a species tank of a single type of very hardy and tight schooling fish in a 20 gallon high. The tank is for my cousin who has never had an aquarium before. Species that come into mind include bloodfin tetras, black neons and harlequins. Which species do you guys think are the most hardy of the three? Any other ideas are welcome. Also, would bloodfins school as well since the tank is only a 20 high? They seem to be the larger tetra in comparison to the black neons and harlequins.