Liquid that does not spoil for a mineral aquarium
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.