Here is just one of many articles addressing this subject.
http://www.pondtrademag.com/articles/ar-184/
Good article.
I was going to facetiously post that if rainwater was such a danger, that all ponds should be covered with a tent, or even built indoors.......I read the article.
I laughed when it suggested that the pond should indeed be covered.
Of course, if you choose to build a pond beside an oil field with flare towers......
Use the rainwater.
Problem is more so with the solvent capability of rain water. Pure water is an excellent solvent, which means it will dissolve anything that has a notable solubility in water.
Sure, if the rain water was directly entering the pond, then most likely no worries. However, whenever
runoff is concerned, there is far more added to the water and, at this point, there is no reasonable approach to suggest the rain water is anywhere near "clean". Run off would involve various captured salts from roof cleaning solutions or from land based fertilizers or could carry other elements. In many areas, roof cleaning is quite common, which involves various chemicals that leave a residue. Depending on the roof tile, there are particular tile that are more prone to UV decomposition thus release the constituent chemicals along with the rain. If the roof is some type of metal or metal-like material, then likely no worries unless roof cleaning is involved. Generally, I always recommend capturing the roof water and allow this to gravity flow through an activated carbon filter, which can be easily DIY'd or can be built into the gutters.
"Clear does not mean clean" for fish use such as look at how the sodium component within baking soda or other pond aquarium products gradually increases the water's salinity over a long period of time, dependent on use, but the water still "looks clear". Same applies with nitrite buildup from runoff even though the water "looks clear".
"Salt" is not specifically referring to sodium. "Salt" refers to any residual compound that remains within water after a chemical reaction and chemical reactions are constant all around us, even within our own bodies, depositing different salts that either must be physically removed or biologically consumed. Salt buildup is the concern when involving
rain runoff water.
I bet rain water is quite safe for us since our air is no where near polluted like it is in Mexico City, but, if you live near an incinerator or other areas that have these smog producing facilities, then there should be concern. The specific reason why mercury levels are so high in particular "freshwater" lakes near cities is due to these smog industries, which is why garbage incinerating was outlawed sometime ago except think there are still exceptions allowed.
Now, this is not a problem for the fish, but, if you have pets drinking from the pond or care to use the water to irrigate vegetables, then the extent of water pollution becomes more of a concern even though the fish might be fine.
Generally, I imagine most folk will be fine, but there are always exceptions to the rule, which should not be so easily ignored.
I was told on one forum that I shouldn't even consider building a pond because I was in an acid rain zone. My plants and fish would never survive. Five years later I'm still waiting for the dire prediction to come true. When my fish finally die of old age, I'll assume it was the rain water.
Yeah, I think there is too much fear mongering when rain water itself is involved, except the situation changes slightly when talking about rain
runoff water such as when it comes from a roof or an over fertilized lawn or a flowerbed contain significant pre-emergent residuals, etc.
I know of folk that have had pond kills due to serious rain falls, which is why maintaining alkalinity is quite important. Aquatic kills due to heavy rain fall in small lakes or rivers is actually not that rare.
Generally, I imagine most folk will be fine, but there are always exceptions to the rule, which should not be so easily ignored.
There are easy fixes such as filtering roof rainwater runoff through a gravity flow activate carbon filter, buildup berms around the pond to prevent rain runoff from lawns, and to increase the water's alkalinity or utilize a significant volume of crystalline calcium carbonate (i.e., crushed oyster shells, other forms, etcs).
Personally, I very much welcome the rain water since I am trying to make my alkalinity quite low and my ground water alkalinity is incredibly high. However, I just don't let the rain water
runoff enter the tank/pond water without proper precautions.
