I agree with the above. That being said, here is a guideline, but i would stll follow recommendations as above also.
http://www.3reef.com/aquarium_calculators/
click fish capacity
Lova ya Mikey, but that calculator ought to be blown to bits. According to it my 120 should be able to hold 29.88" of fish..in length. That is absolute nonsense . (Admire your intent though!!
)
First off - when trying to figure out how many fish you can safely put into a tank you need to take their ADULT SIZE into consideration..NOT what size they are 'now'.
If you put a fish that should ultimately grow to 9" into a tank that is too small to accomodate their adult size, their body will still
attempt to grow to it's proper size but it may fall short and be
slightly stunted. It is a myth that a fish will only grow to fit the size tank it is raised in. What happens is some part of the fish, often one or more of its internal organs will not attain full size which will reduce not only the quality of it's health but also the length of it's life.
Secondly, and I'll give you an example for this one...
I have 2 fish in my tank that are both about 4 1/2 inches long..
Here is one of them:
Here is the other:
Huge difference in body mass. That must be taken into consideration too!!
Always pre-plan, know what you are buying and you should be fine. Don't go by 'inches', use mass instead. The lighter you stock, the easier it will be to maintain your tank