Can you house a blue hermit in a 2" cube with a pinch of sand? To change the water I'll just take a small bit from my current tank. The only problem is how would i heat it? Then I'll sell each cube for like 20$ when it costs so little!! Muahaha!!
My first thought is "would you like to be confined to your shower for all your days?"
Second, any evaporation would result in large salinity fluctuations. This would be incredibly stressful to any invertebrate, if it didn't end up killing it.
As hobbyists, we should be promoting the general well-being of the critters we choose to keep. I just don't see keeping a hermit crab in a 2" cube as promoting its well-being.