I use Reverse Osmosis water which I pick up locally from a water supply company and then I remineralize it for my tanks. I have well water here and use potassium chloride to soften it. I do have one outdoor hose that bypasses the water softener but we have a high amount of iron in the well water. I would rather use āpureā water and remineralize. Things were so simple when I lived in the city and used city water and the water conditioner.
If you use rainwater it is possible other things could be in the water from the atmosphere such as pollution, soot from fires, any aerosolized bits that are in the air. You also need to consider your collection methods. If youāre using a downspout from gutters around a building, are those gutters clean? Are there any metals leaching into the water? Where are you collecting the water? Is that collection container clean? Giardia is a water borne illness and could enter the water from an infected birdās droppings. I donāt mean to sound extreme, but there are also things to consider when using rainwater also.
Rainwater is also soft water and slightly acidic, and whether or not that will suit the types of fish you are keeping is another thing to consider.
In my opinion, I look back on my city water days as āthe good old daysā before I had to mix powders and test water each time I do a water change. It was kind of nice just getting water from the tap and adding a small amount of the conditioner and that was all.
Now, you could just let your tap water sit 48 hours before using it in your aquarium to let the chlorine evaporate. You could test it to see if any chloramine is still there. Aquarium Co-Op sells test strips that include chlorine and chloramine indicators.
You could also boil the water which will remove potential pathogens and bacteria. Of course you would need to let the water cool to the proper temperature before adding it to your aquarium
You should run the rainwater through a carbon filter anyhow before using it in your aquarium and doing so can help remove chloramine. Chloramine exhausts carbon quickly though so you would need to replace your filter carbon frequently. Again, using a chloramine test strip can help you determine when.
So I am not so sure that using rainwater is the easy answer either. To me, like I said, just use the water conditioner and be done with it.