Rei,
If all your paramters (NH3/NO2/NO3) are at the appropriate levels then I would recommend that you purchase an air pump. You don't need anything fancy, just something that pumps about 10-20L/hr more than the size of your tank. The reason why I say this is because the aerobic decomposition of organic material/waste are part of the biological oxygen demand (BOD or "bioload"). The metabolic decomposition of such matter generally use more oxygen than respiration itself. Having said that, your axolotl is most likely (if it's not because of concentrated levels of NH3/NO2) gasping at the surface of the water because of "oxygen starvation". Keep in mind that it is not the low oxygen level but the siginificantly high carbon dioxide (CO2) level within the water. Without a sharp gradient between CO2 levels in the blood and CO2 levels in the water, it becomes increasingly difficult for axolotls to diffuse the CO2 out of their blood. And with the bacteria in your tank using up more oxygen than your axolotl, and releasing CO2 as a waste product, it could explain why your axolotl is gasping.
On another note, the gasping at the surface could be related to high nitrite levels rather than an inadequate oxygen supply. I say this because nitrite has a greater affinity for the blood's hemoglobin (Hb) binding site that would ordinarily be transporting oxygen. This works the same way with humans. Carbon monoxide has a greater affinity for iron (Fe- the binding site) within the Hb molecule, which is why during a fire when we inhale the smoke we eventually pass out or die due to carbon monoxide poisoning.
Another thing that you should keep in mind is that there is obviously oxygen present in the atmosphere, so the dissolved gases within your tank will try and reach an equilibrium with the gases in the atmosphere. For instance, there will obviously be more oxygen in the atmosphere than within your tank, therefore oxygen will diffuse down its partial pressure gradient (that is from an area high oxygen level to an area of lower oxygen level). But it will never reach an equilibrium because the of the BOD within your tank. Hence, the air pump will come in handy.
Also, just wondering if your keep any plants?
Jay.