Can you fill an aquarium with nitrogen? My goal is to create an environment of nearly pure nitrogen where only anaerobes can survive. I need to be able to move samples in and out of it. How would you fill it?
You can fill an aquarium with CO2. It is a common demonstration to fill it with smoke or fog and pour CO2 into it. Could you invert the aquarium and fill it with nitrogen?
My thinking is:


*

*There isn't enough buoyancy for it to rise and displace the oxygen.

*You would only produce an aquarium with a slightly higher percentage of nitrogen. 

 A: I have tried this in my Bachelor thesis, and found that it works - but the aquarium needs to be upside down, since the density of nitrogen ($1.165~\text{kg}/\text{m}^3$) is slightly lower than that of ambient air ($1.205~\text{kg}/\text{m}^3$).
I used a box that is open at the bottom that is slowly filled with nitrogen from the top, in order to displace the air.
More interestingly, the natural vapor pressure of evaporating liquid nitrogen is enough to force it through the tube and fill such an aquarium. There is no need for a pressurized mechanism.
Of course, this is quite sensitive to air movement outside - since the densities are nearly equal the gases tend to mix easily and over time diffuse into each other. It is therefore important to continuously supply nitrogen.
A: Do you mean an open aquarium? It won't work, for two reasons. 


*

*Density of nitrogen is slightly lower than density of air in the same conditions (air consists of nitrogen + oxygen which is more dense + traces of some other gases which are even more dense + humidity which is light and compensates the effect to an extent). The difference is not large, yet nitrogen will tend to move upwards off the tank. 

*Gases are prone to diffusion, ie. natural mixing caused by molecule collisions. Small difference of density between pure nitrogen and air only supports this phenomenon. So do air movements caused by differences of temperature in various parts of the room or by moving objects. It's only a matter of time for oxygen to fill your aquarium in a proper proporion. 
Conclusion: you can fill your aquarium with nitrogen, but it will be effective only if you refer to a hermetically closed tank.
