Liquid that does not spoil for a mineral aquarium I want to create a decoration - a mason jar "aquarium". I want to take a large mason jar, put some nice mineral specimens in it, fill the jar with liquid and seal it with lid.

I believe I can't use pure water because there is a chance that it will go bad after some time.

So I want to know if there is some liquid that is not too aggressive (in terms of mineral decay) or dangerous that has indefinite shelf life. I want it to last for years.

I know that vodka basically never goes bad, but it's pretty expensive and the alcohol would probably be too harsh for the minerals in the jar.

Additional information: I am considering water with salt.
There are three things to take into consideration here: volatility of the liquid you will be using (yes, the jar will be tightly closed, but some candidates might still evaporate slowly and/or have toxic or foul-smelling vapors), stability, and refraction index. Mineral oil is a great candidate, as Todd Minehardt commented. It's really stable, releases no vapors whatsoever, but its refractive index is a little higher than that of water, which might make your aquarium look funny. There are substances with similar properties that might be useful and also readily available, but with a refractive index closer to water's. Amyl alcohol (fusel oil) or ethylene glycol (car antifreeze) are good candidates and shouldn't be too hard to find. Glycerol and propylene glycol are also pretty good options, but their refractive indexes are closer to mineral oil than the other two.