What size aquarium do I need

To build a better and larger aquarium, one needs to know the minimum required size of aquarium for any type of fish.

Defining the minimum size of an aquarium for fish of any type:

I have three fish of adult age of a certain size
 that might be small or might be large.

  Two of them are girls, of that I'm sure. 

The third is a male with much allure.

 I love my fish!

I want my fish to live in one tank. 
And for them to have anything they can think (of):
 water to drink and splash around in and swim;

 food to eat;

  a heater; 

filters;

water plants; 

air to breath and to bubble like a jacuzzi.

  They might have babies in the future!

 Though, never mind those little creatures -
 I'll drop them off at a school to fool around. 

 And you should know that fish swim

forth and back, 

up and down, 

in all directions! 

I have an idea that could be wrong.

 From what I know, if I am wrong my fish may die.

 And if I am right...I will let you know!  

I have to build a tank the size of...-

 I am asking you to figure it out!

(3 * L)^3 * X + R = V
L - size of the fish 
X - number of fish 
R - as much space as you can add
 V - the best aquarium ever

(3 * L)^3 = M
L - size of the fish
M - The minimum size aquarium for one fish

 An example. 

(3 * 5 cm)^3 = 3.375 liters is the minimum size aquarium for one 5 cm long fish.

Defining the minimum size aquarium for fish kept in large aquariums:

Get a pair of fish in a large aquarium.

 Hooray! The fish have babies! 
Keep all fish in the same tank.

  As years go by, fish keep multiply! 

Count adult fish each year.
 At the beginning, the number goes up.

 Then goes down and up again.

 But it never goes over a certain number.
 Use this number to calculate the minimum size aquarium for one fish.

V / X = M
V - the best aquarium ever
 X - the maximum number of adult fish kept in the best aquarium ever. 
M - the minimum size aquarium for one fish. 

An example.
 If the number of adult fish that lives in 150 liters aquarium for many years was about 50 than 150 liters / 50 = 3 liters is the minimum size aquarium for one fish.

It would be interesting to see how this correlates when calculated in both ways. There should be other ways to define the minimum size aquarium for any fish. Please share your ideas! 

Either way the calculated minimum size aquarium should be used as a baseline, a starting point, for building better aquariums. Fish behavior, habits, and many other factors should be taken into consideration for building an aquarium for any particular type of fish.
 Have fun and happy fish :)