I wouldn't use rain water if you live in a built-up area, at least not without filtering it first. Tap water with dechlorinator added is what I use for my discus, with two 30-50% waterchanges weekly, though they will need more regular changes untill they reach 5 inches. I clean one of my two filters each month
Rain water in a build-up area will have collected polution from car exhausts, factories e.t.c on it's way down to earth, and could be just as bad as river water. Tap water is strictly regulated and won't be harmful. There are no regulations on rain water, and very few on river water. Tap water is your lesser of three evils
If you must use rain or tap water, run it through three stages of filter. Floss to remove sediment, carbon to remove polutants and a UV to remove parasites. The flow through your DIY filter will need to be slow to ensure that each stage is at it's most effective. To give an idea, for me to be comfy doing a 50% change on my own tank with rain/river water, I would want all 150l to pass through the filter in two
or more hours to be sure that the water was safe. On top of filtering, you then have to add minerals before use. IMHO, it's too much work for NO real benefit.
Discus don't need a low pH unless they are wild caught. Domestic discus aren't likely to have been raised in water with a pH lower than 7, and my tank runs at about 7.5
HTH
Rabbut