It is the natural thing for any population to max out in its environment (provided that there are enough resources). The population will (theoretically, if enough resources are present) go above what is called the carrying capacitay, or what biologists call "K
". The carrying capacity is the number of a certain organism that an area, an aquarium in this case, is capable of SUSTAINING. This is the key word. The population will increase beyond this threshold level and then organisms, bacteria, will die off due to competition for resources and approach "K
" where the population levels off and becomes stable.