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One way that you can provide suitable water for growing Disas is to use a reverse osmosis water purification system.  This will provide you with water that approaches  a TDS of 0 ppm.  The only problem is that there are no salts or nutrients whatsoever in the water and you have to add absolutely everything that the plant needs. Since the cost of RO units has been decreasing, if you have hard water and want to grow Disas, then this is the way to go.  RO water isn't cheap though so one strategy would be to try and collect rainwater off a clean plastic surface for normal use and then use RO water during the dry season. BTW, I was once asked what to add to RO or distilled water so that the plant's needs were met.  The simplest answer is a small amount of your tap water.  Just make sure that the reading on the TDS meter is no higher than around 30 to 50 ppm  for the mixture.