Persephone,
Excerpted from our web site (Culture page under Water and Fertilizer)
"...if your tap water contains more than 300 parts per million (ppm) of dissolved solids, you will be limited as to the varieties that will grow optimally. Adding fertilizer to water will drastically increase its amount of TDS; therefore, less fertilizer is required in water that already has high quantities of minerals in it. Flushing with plain water is more important when high TDS water is used in regular watering. Plants watered with high TDS water may benefit from being grown slightly more moist since dry roots are more likely to be damaged when higher amounts of minerals are present, but be sure to monitor the freshness of the potting mix to prevent root rot."
That 300 ppm is for orchids in general, and Slippers are even more sensitive (with exceptions) as the root hairs are easily burned by excess salts. When our collection was small, we bought water at the grocery store by the gallon. If this isn't feasilbe for daily watering needs, at least get some to flush once a month or so. Avoid Spring Water as it usually has minerals you're trying to avoid. Get RO, Deionized, Distilled, or similar. If you take your own containers (bleach milk jugs OR boost an empty 5 gallon water cooler carboy from work- I didn't say that- OR buy a container at WalMart in the camping dept), lots of stores have self-serve water dispensers that brings the cost down to about $0.30-0.40 per gallon versus about a buck a gallon for "pre-packaged". Home RO units are fairly affordable at about $100-$200. Keep in mind RO units waste several gallons (~2-5 depending on temp, pressure, input water, etc) of brine for each gallon of good water produced. Could jack up your water bill. DI is chaper up front BUT more expensive in the long run due to cartridge replacement or recharging, but has no water waste.
-Ernie