Water quality is one of the more important aspects of growing almost any kind of plant and especially in growing orchids. Although they are in general fairly hardy plants, orchids seem to react to hard water worse than many others. Some will grow in hard water, but others will simply sulk. Softened water will kill most orchids, so if you have a water softener only use bottled water or water that has not gone through the system.
The best water, of course, is rain water -- it is inexpensive and environmentally friendly. For a small collection itís possible to use bottled water. Nurseries usually use reverse osmosis (RO) water, but for the average grower this is quite expensive. Hard water from a hose can be filtered through a calcium filter which will help reduce the amount of minerals reaching your plants. If you use any of these demineralized waters though, you will need to use a fertilizer with trace minerals, or water will unfiltered water about once a month to replace minerals lost through the filtering process.
There are many advantages for your plants to using pure water. If you have naturally hard water where you live, you will find that misting with it will eventually cause a white buildup on the leaves which is not only unsightly, but keeps the leaves from breathing naturally and does not allow the sun to reach them. The leaves can be cleaned with a lemon juice wash, but if you have very many plants, this is a real chore. Species orchids seem to resent this buildup more than the hybrids, but they all perform better when watered with purer water.
There also seems to be a positive reaction in root growth, particularly among those orchids that have air roots when the water quality is improved. It is not apparent to the eye, but my experience has shown that plants seem to grow better and produce more flowers when the water quality is changed from very hard water to rainwater and monthly watering with water from a hose with a calcium filter.
When collecting and using rainwater, be sure that you either dispose of any standing water or cover your container so that you do not encourage mosquitoes. They will lay eggs in any kind of water left uncovered. A suggestion is to use a container with a cover and only uncover it when you expect rain. We use an old garbage can under a rain spout for this purpose.