Water and detergent (or something that contains a surfactant) are the key ingredients in bubble solutions. Despite what you may read in various recipes on the internet, tap water is generally as good or better for making bubbles as distilled or reverse osmosis water.
The minerals in most tap water is beneficial to sturdy soap films, and most modern detergents were formulated to work with tap water.
When I began exploring bubbles in 2010, there were a lot of disputes about whether tap water or distilled water is the best to use. Since that time, many people have explored whether their recipes worked better with tap or distilled water. Very very very few people have found that distilled water improves their mixes. In fact, most people found that tap water worked better for them than distilled water. This group of people includes people that had been using distilled water without ever having tried tap water because their tap water is very hard.
There are rare cases where distilled water works better than tap water, but even in those cases a little bit of tap water in the mix seems to improve performance.
Most recipes are based on high-quality dishwashing liquids that have been formulated to work with a large variety of water hardness and pH, it is likely that distilled water will only be necessary in locales where the water has very particular (but as yet to be determined) qualities such as extreme hardness or alkalinity or pH.
What Should I Use
You should try tap water first. Choose a reputable recipe. Follow the recipe precisely. Make sure that you follow the directions carefully and do everything in the order mentioned in the recipe. I cannot tell you how many times (countless) I have helped debug bubble juice problems that they attributed to water that were due to their not following the directions provided with the recipe.
If the conditions were good and your equipment was good and your results were disappointing, mix up one batch of juice with tap water and one with distilled water. Try both batches in the same session. After comparing tap and distilled water, add some of the tap water mix to the distilled water mix. You should compare the resulting solutions side-by-side or in the same session since changes in atmospheric conditions can have a huge impact on the results. For example, don't mix up a tap water solution one day and try it and mix up distilled water the next day. The difference in performance is more likely to be due to changing conditions.
Professionals and Water Quality
Since water quality (hardness, pH, total alkalinity, salt content and particulate content) can vary dramatically from locale to locale, some professionals that need to mix up their solutions in different locales use distilled water. One of them explained that while the local tap water might give better results than distilled water that he requires consistent behavior from his mixes and that different tap waters give different 'feels' to the mix. For him, using distilled water gives him the same results no matter where hs is.
On the other hand, I know professionals that use tap water and only switch to distilled if the tap water does not work.
A few people report using particular brands of bottled (non-sparkling) mineral water wherever they go.
Anecdotes
There are unsubstantiated claims that a large Chinese manufacturer of bubble juice imports large amounts of Lake Michigan water to use. Regardless of whether this claim is true, there is no doubt that the characteristics of the water used influence bubble solutions.
In an informal interview quoted on the web (need reference here), the chemist behind Cricket Hill Powder (a widely-admired product for making big bubbles) indicates that tap water is generally preferable to distilled water as there are minerals in tap water that are important to good bubbling.
pH
Water pH can vary considerably. Tap water pH can run from about 5.0 to over 9.0. This may account for some people's preference for distilled water. Most bubble juice works best in a particular pH range. Bubble juice pH can be adjusted without requiring distilled water. See the pH article for more information about this very important topic.
Distilled Water pH. Contrary to what you might think distilled water does not usually have a pH of 7.0. Distilled water that has never been exposed to air will have a pH of 7.0. However, air contains carbon dioxide which is soluble in water. Dissolved carbon dioxide creates carbonic acid, a mild acid. Distilled water tends to have a pH somewhere between 5.5 or 6.5 due to dissolved CO2.
Hardness
Hardness is a measure of dissolved mineral content. Typically, water hardness refers to the amount of dissolved calcium and magnesium. Hardness sometimes refers to Total Hardness (the combined concentration of calcium and magnesium ions) and sometimes calcium-hardness (the concentration of calcium ions only).
Hardness is expressed in a number of units or scales, including:
- parts per million (ppm, mg/L, or American Degrees)
- grains per gallon (gpg)
- degrees of general hardness (dGH), and more: https://en.wikipedia.org/wiki/Hard_water#Measurement
For more information about water hardness, see the Wikipedia article
Water hardness is generally not a problem when brewing bubble juice with the recommended dishwashing liquids. However, in cases of very hard water, there may be a negative effect on detergent effectiveness.
Water Hardness and Guar Gum. Calcium and borax are known to cross-link with guar gum suspensions. While guar gum is very sensitive to the presence of borax (even a very low borax concentration causes the guar gum to settle out as as a slimy layer), calcium occurs in low enough concentrations that it is generally not a problem when making guar gum-based bubble juice with tap water. However, it is possible that the calcium concentration in very hard tap water is high enough to be implicated in the formation of guar "sludge" that some people experience when making guar-based bubble juice. See the guar gum-based recipes page for more about sludge. See also, the Wikipedia Guar Gum article.
July 2015: Wiki member SpaceCaseChris is exploring whether water hardness is responsible for sludge formation.
Oct 2018: Edward's interpretation SpaceCaseChris' explorations is that hardness is not implicated in 'sluge' formation of guar gum solutions.
See Also
Post by water-chemist John Pastorello sharing his thoughts about water.