well a little knowledge i can provide about keeping guppies.. Most of the vendors says u can keep guppies without air pump.I disagree with that.It actually depends on the size of ur aquarium.If u are having a deep tank , it is advisable to have an air pump.It is not that guppy dont need oxygen to breath in, but they get their oxygen by swimming to the top.So juz imagine that it have to swim up to get some oxygen and then down.By doing their rather frequently, it will be rather exhausting.