Try fertilizing every third or fourth watering with Minor (Trace) elements along c your N-P-K (Nitrogen, phosphorus and Potassium) fertilizer, 1/2 strength for Tillansias and houseplants.
Trace elements include Magnesium, Iron, Manganese and other trace elements.
And you could add a Brita filter on your tap. This will take a lot of hardness from your water without stripping it of essential minerals.

Water softening filters soften water by replacing the calcium with salt which would still leave a mineral deposit in your terrarium. You would need a reverse osmosis system or distiller to remove the minerals without adding something else to the water.

I would use a hydroponic nutrient solution mixed in the distilled water for what you want to do, 1/4 strength. The reason for recommending this is that these nutrient solutions are designed to provide everything your plants need including micro-nutrients and essential elements that often aren't found in regular fertilizers.

Thank you dellis and leafhead!

dellis,

Just before I saw your post I had thought about the distilled water and the hydroponic nutrient solutions combination. I guess great minds think alike! lol Thanks for reinforcing my idea.

Oh, by the way, the RO systems I looked at do not remove calcium.

Thanks again,

Larry

Any RO unit worth it's weight in salt, should be taking out the majority of everything and if it contained a DI stage there should be nothing in the water exiting the filter system.

Here's the best bang for your buck that will produce 0 ppm water all day long and it's a quality unit with quality components you could use for many years with only filter/ resin replacement/maintenance.

http://www.bulkreefsupply.com/brs-4-stage-value-ro-di-system-75gpd.html

Here is the full listing of filtration systems offered by BRS. If you/anyone has any questions you can email them and their customer service (which is about the best I have ever delt with) will happily and promptly answer your questions.