housing in 2 gal aquarium

[Oxylepy]

You need a certain amount of surface area to oxygenate the water. Bigger fish need more surface area.

Do you mean surface area or volume? Filters are supposed to oxygenate the water, and so are those bubblers and how much oxygen the fish gets depends on the volume and oxygen content of the water. With adequate filters/bubblers then the water will have the right percentage of oxygen in it, without the need for a high surface area on the top (the only direction that is exposed to air). However low volume means the fish wont really be able to do ram ventilation and buccal pumping will deplete the oxygen content of the water faster.