Eeeeemmmm true in a sense. The transfer of oxygen between atmosfere and water depends on the interface between them. The larger the surface the better the interface. Therefore keeping guppies doesn't require you to have very big deep tanks but instead large surface area shallow tanks. I even had guppies in my outside basins planted with water lilies which the experts won't agree because the leaves covered much of the surface until the haze wiped them out because of the layer of dirt deposited. Anyway having airstones will help the aeration by creating turbulence on the water's surface. You may not agree to this documented findings I about to reveal. The bubbles created by the airstone inside the water doesn't actually provide the dissolved oxygen. Why you may ask. This is because the bubble actually rising at a rapid rate that actually prevented it to be dissolved into the water. It was actually the motion it created on the surface like a mini tsunami hehe waves on the surface when it reaches the surface and burst that actually helps the aeration of water. So if you have a filtration system that creates little waves on the surface water it should suffice.