Rainwater or tapwater?

I am not sure how large your aquarium tank is, but if it is not over 55
gallons there is a good option if you have enough buckets.

If you have strong chloramine problem in your tap water you can collect
your tap water in 5 gallon buckets and let it sit for 48 hours for the
chloramines to evaporate. If you have an extra powerhead or header
these would speed up the process a little bit.

You could also use a chloramine remover that can be found at your local
pet store. This way, you could just introduce the tap water into your
tank.

The advantage of this is that you get the minerals that are found in
your tap water which is important for the hardness of the water as well
as for aquarium plants. Rain water does not contain these important
minerals.