Rain water makes a great mix with tap water to reduce the hardness and TDS of your water, Aquabaz. The thing is knowing what water you are starting with so that you can get your proportions right. For me, with my particular water, I use 3 parts rain water to 1 part tap water for the sensitive soft water fish that I keep. When it is too cold outside to collect rainwater, I end up using my RO water for the dilution. Basically you need to start out knowing what the tap water has in it, in terms of mineral content. Then you decide what the target value would be for the sensitive fish that you are trying to accommodate. Once you have both of those parameters, just figure the mix it will require to bring the mineral content down to where you want it. For my livebearers, by far the biggest group of fish that I keep, the straight tap water with a pH of 7.8 and a TDS over 300 ppm works fine. I try to run the soft water fish at less than 100 ppm, which means almost the same thing as less than 5 degrees of hardness. A 2 to 1 ratio is not enough to get me there but a 3 to 1 mix gets me into the target range quite nicely. Fiddling about with a 2 1/2 to 1 ratio is just not the kind of thing that I will do. I like the KISS principle, Keep It Simple Stupid.