Depends what fish you are keeping. Neon tetras and others like soft water, i use rainwater for them.
My main tank i was using half rain half tap, as my tapwater PH was way high, i did it that way rather than use buffers. But now the tap PH has dropped somewhat, so i just use treated tap.
rainwater is relativly clean, soon is intranced by air pollution and the rocks it comes into contactw tih. If u live in a limestone area, your tap water is likely to be hard but soft if sorrundeding hills are sandstone or granite. Local water companys do modifying prevailling tap water too.
I've certainly not had any harm come from using rainwater. Though some fish need hard water... So it really depends what type you are putting in it.
My rainwater gets collected from my house roof, so it would only come in contact with any pollutants that may be present on my roof, not really much different to the pollutants rainwater collects on its way to becoming your tap water *L*
Last edited by Alasse; October 16th, 2007 at 07:03 AM.