I think this goes here but if not Mods please feel free to move. I found this on my dog forum of all places and it is very helpful. I think it should be sticky-ed.
Its a stocking calculator and quite accurate. The creator is on my dog forum and is constantly making it better and more accurate.
Many people get on here and want to know what kind of tank mates to get and don't think about what their stocking percent is but that should be a major factor. Always shoot for less than 100% stocked.
Here's the link: http://www.aqadvisor.com/AqAdvisor.php
It tells you how stocked your tank is. What % water changes you should do (although I think they should be greater). Its a great way to know what tankmates you can and cannot have and how many you should have per volume.
My 10 gallon is only 68% stocked with two male bettas and two mystery snails. I can add 10 ghost shrimp and still only be 75% stocked. However my 3 gallon is 131% stocked with a male betta and a mystery snail. I already knew this so I do 3 weekly water changes and vacuum twice a week.