(From a previous post)
A little about water: All water basically starts out as pure H2O vapor in the atmosphere. It condenses around a nucleation point, maybe a dust particle, forms a drop, and eventually falls to earth or evaporates and starts over again to form another drop. There may be some airborne acids & other compounds that dissolve in it, but it's rather pure stuff. Essentially, at this point it is almost the same as distilled water, and because R/O water is usually extremely pure, we can include that in the discussion.
Water passes into plant cells by a process called osmosis. What drives osmosis is the difference between the concentration of solutes in the water that is inside cells compared to the concentration of solutes in the water outside cells. Water with low levels of solutes moves toward water with high levels of solutes until the level of solutes equalize (isotonicity), then water movement stops.

What does this mean to plants? Lets start by recalling what I said about how water moves. Since distilled and rain water have essentially nothing dissolved in them, they pass into cells with the least amount of resistance (or difficulty). All tap water is chemically different, so we cannot make the generalization that 'tap water is ok' based on the experience of 1 or even 100 growers. It may be, but it may not be.

Highly variable in tap water are pH, alkalinity, hardness .... Important among the properties is what is dissolved in the tap water. Magnesium (Mg) and calcium (Ca) are very common in water. Iron (Fe), sulfur (S), and other metals are usually present as well. I mention this because all these things dissolved in water are the same type of salts that fertilizers are made of.

We can't even say that in all cases rain, distilled, or R/O water are best, unless we know what fertilizer the person is using. Most fertilizers that are soluble in water contain NO Ca or Mg. If we use distilled, rain, or R/O water with these fertilizers, we could run into deficiencies of these two elements unless we supply them. Usually, they are in the soils we use from a bag, but if we make our own soils and don't make the effort to supply them (garden lime or gypsum + Epsom salts), deficiencies of these elements are probable.

Tap water can present another problem. Often, there are very high amounts of either Ca or Mg in the water. If the ratio of Ca to Mg gets skewed (3-4:1, Ca:Mg is a good ratio), it can affect the ability of the plant to take up the other element (an antagonistic deficiency). If for instance, your tap water has more Mg than Ca, you are probably going to see evidence of a Ca deficiency. If there is a fair amount of Ca, but almost no Mg, it's likely the opposite can occur. These deficiencies are common and can easily occur when we add Epsom salts (Mg+S) w/o adding Ca.

One other important way that tap water affects plants (besides pH): As the level of solutes in the water rises, it becomes more difficult for the plant to take up water and the nutrients dissolved in it. It's always an advantage for us to keep the level of solutes in the soil solution low (especially in winter, when it's important for plants to absorb water efficiently). Let's say that we have an imaginary plant that will start to struggle with water uptake when the level of solutes reaches 1,000 ppm. IF we start out with tap water that already has 300 ppm dissolved solids in it, we can only add 600 ppm fertilizer, where with distilled water, we could have added the full 1,000 ppm.

Why is this important? Well, if your plant needs the equivalent of 250 ppm N between doses, and you're using a 1:1:1 ratio fertilizer (like 20-20-20), that means that in order to supply the 250 ppm of N, you have to supply 250 ppm of both P and K as well (because they are found in equal measure in the fertilizer). That means you would supply 750 ppm of fertilizer to satisfy the need for 250 ppm N. When you add the 750 ppm to the 300 ppm in our tap water, suddenly you're at 1,050 ppm in the soil solution w/o even considering what residual fertilizer and dissolved salts are left in the soil before we fertilized.

I'm not using the illustration to say you should expect trouble with tap water, only to show that you need to be more careful .... about your rates, your soils, and your choice of fertilizer blend %s. In the example above, we could have supplied 24-8-16 fertilizer at only 2/3 the rate of the 20-20-20, which would have satisfied the N requirement, provided more than adequate P&K and brought us in well under 1,000 ppm.

If you use a fertilizer that supplies all the nutrients a plant needs in a favorable ratio (I'll post a chart) in rain, distilled, or R/O water, you can have the lowest concentrations of solutes in the soil solution possible, while still supplying nutrients at above deficiency levels. I most often use Foliage-Pro 9-3-6 because it has all the essential nutrients, including Ca and Mg, which are rare in soluble fertilizers. Plants do 'take what they need and leave the rest', but it makes no sense to knowingly supply nutrients the plant cannot use, because those unused nutrients add to the concentration of solutes in the water, increasing the solutes in the soil solution and making it more difficult for plants to absorb water & other nutrients.

The chart:
I gave Nitrogen, because it's the largest nutrient component, the value of 100. Other nutrients are listed as a weight percentage of N.
N 100
P 13-19 (16) 1/6
K 45-80 (62) 3/5
S 6-9 (8) 1/12
Mg 5-15 (10) 1/10
Ca 5-15 (10) 1/10
Fe 0.7
Mn 0.4
B(oron) 0.2
Zn 0.06
Cu 0.03
Cl 0.03
M(olybden) 0.003

To read the chart: P - plants use 13-19 parts of P or an average of about 16 parts for every 100 parts of N, or 6 times more N than P. Plants use about 45-80 parts of K or an average of about 62 parts for every 100 parts of N, or about 3/5 as much K as N, and so on. When you see that plants use about 6X more N than P, and not quite 2x as much K, it makes little sense to apply 20-20-20 and give them equal amounts of each.

I know it was a long, long way to make the point that distilled/rain water are best if you want to use a fertilizer complete with all the essentials, but in the end, 'what kind of water you should use' depends in very large part on what is in it, how much of it, and in what ratios to the other elements that are also in it.

While tap water is probably perfectly fine for those of us who know how to water properly and do so, it can present problems more quickly and more severe for those using heavy soils that require watering in sips to prevent root rot. If you are one of 'those' waterers, then you will find your plants will fare much better if you use rain water, distilled water, water from a dehumidifier, or snow melt. I use tap water for almost all my plants, but a very few, because of unusual circumstances, do get special water.

Al