This is a very common question posted on message boards all over the internet and we are all guilty of the urge to add just a few more fish to a tank so I thought that I would write a composition of every thing I have observed and learned to help some people know what to watch out and test their water for. It is a very long winded answer I really don’t know how to shorten it. Every time someone asked this question you would get different answers.

Over the years, a few " laws" have been developed depending on what type of fish you are keeping. The most common for freshwater tropical fish is the "1 inch of fish per gallon of water," (2.5 cm of fish for every 3.8 liters) a variation of which is the "one inch of fish per 12 square inches of tank surface area"( 2.5 cm of fish for every 30 square cm of surface area) . In reality, these laws are not very useful except for as a rough guideline. How do you compare a 6" long aggressive Cichlids  with a 2" long platy?  Body mass, rather than length, and aggressiveness should be taken into consideration.

The most important limitation in your aquarium is Oxygen. Fish must have oxygen to breathe. The more fish in the water, the more oxygen that must be supplied. When Oxygen reaches total saturation then there is no more Oxygen available , no matter HOW MUCH you try to pump more into it. Fish are not the only users of this limited Oxygen. The more Fish to feed and make waste the more bacteria need to grow and take care of that load. This added load of bacteria is also part of the total bio load in your aquarium. Gas exchanges at the surface of your tank made with agitation has proven to be the best Oxygen source and gas elimination for an aquarium. Temperature and elevation are very important factors in determining O2 saturation levels. Higher temperature and higher elevation mean less available O2 for fish and bacteria.  Get an Oxygen Test if your fish are gasping. Test BEFORE or just after you turn the lights on for the lowest reading in a planted tank.

For further details on Oxygen and it’s importance you should read this very detailed paper and note the Saturation levels.. http://www.bioconlabs.com/oxy.html

Ammonia is also a limiting factor. Ammonia is removed by nitrification (bacteria) to form nitrite and then nitrate. ( It can also be removed directly through plants.)  This process produces free Hydrogen ions which in turn also will lower PH. At a lower PH the Bacteria slow down and this can lead to an ammonia build up which in turn will lead to stress for your fish .The rate at which the pH drops depends upon the KH ( buffering ) of the water and how much nitrification is taking place. Over feeding can make the Ammonia / bacteria levels rise.