that's right -
there is a limit to the amount of dissolved oxygen in the water
as psipro pointed out...
when the water is saturated, no more oxygen can dissolve in the aquarium regardless of the amount of the airstones or surface agitation.
on this same note:
because there is a limit on the amount of oxygen that can enter the aquarium -
this in turn also limits the stocking/biocapacity of the tank.
Good bacteria require oxygen to breed.
Therefore, there will be a stage where the aquarium can no longer support both the fish and bacteria (because both require and will compete for oxygen)
Another point is regards to the airstone -
although the airstone will help maximize the amount of oxygen in the aquarium - you have to keep in mind the water movement in the tank
some fish do NOT like excessive bubbles and current in the tank -
1. make sure that there is a place for the fish to rest
2. make sure that the current is NOT at the center of the tank (if possible, make the current on the side). This will maximize the swimming room in the tank
