I should tell you that putting an aquarium outside is usually a pretty bad idea, I think. Make sure it is at least heated. Aquarium plants are always good but they breath carbon dioxide only when they have light, when it is dark they breath oxygen. So during the day they will help but at night they will take away the oxygen. The specie of fish really doesn't matter when your talking about oxygen levels. Like a dog will suffocate the same as human, different type of fish will suffocate just the same. You will have a terrible problem with algae if you put in sunlight. That aquarium will turn really green really fast. A surface ripple is the absolute best way to oxygenate the water. The level doesn't have to be low enough to make a splash, just low enough that it makes the water look rippled at the top.