Is it safe to use rain water in the aquarium? There is a lot of debate about this for some reason which I can't understand. The simple basic fact is that all fresh water fish live in rain water without exception. It is also worth keeping in mind that there is no possible way for the rain water to become cleaner or more pure once it hits the ground, It is true that in heavily industrialised areas or areas near to major road networks MAY have airborne pollutants some of which may be soluble. But this is a problem which is easily overcome by not collecting the water from the first thirty minutes of a rain storm so that the majority of the pollutants will be washed away.
The method of collection of rain water is important and will determine in part the quality of the water.
Rain water can be used to replace evaporated water without the need to add anything to it at all.
It can be used to make regular partial water changes but it must be treated in the same way as r/o water and it should be re-mineralised before use or the system may eventually suffer from a pH crash.
Finally rain water can be used to soften hard tap water simply by mixing the two together in the right proportions to achieve the desired hardness before adding it to the aquarium.
There is obviously no need to use a dechlorinator with rain water because it doesn't contain any chlorine or chloramine